Add UndoStack class - a custom undo/redo engine

It will be hooked up to the markdown editor later

16 files changed, 1287 insertions, 39 deletions

diff --git a/app/assets/javascripts/commons/polyfills.js b/app/assets/javascripts/commons/polyfills.js
index 7a6ad3dc771..daa941a63cd 100644
--- a/app/assets/javascripts/commons/polyfills.js
+++ b/app/assets/javascripts/commons/polyfills.js
@@ -12,6 +12,7 @@ import 'core-js/es/promise/finally';
 import 'core-js/es/string/code-point-at';
 import 'core-js/es/string/from-code-point';
 import 'core-js/es/string/includes';
+import 'core-js/es/string/repeat';
 import 'core-js/es/string/starts-with';
 import 'core-js/es/string/ends-with';
 import 'core-js/es/symbol';
diff --git a/app/assets/javascripts/gl_form.js b/app/assets/javascripts/gl_form.js
index a66555838ba..b98fe9f6ce2 100644
--- a/app/assets/javascripts/gl_form.js
+++ b/app/assets/javascripts/gl_form.js
@@ -3,9 +3,16 @@ import autosize from 'autosize';
 import GfmAutoComplete, { defaultAutocompleteConfig } from 'ee_else_ce/gfm_auto_complete';
 import dropzoneInput from './dropzone_input';
 import { addMarkdownListeners, removeMarkdownListeners } from './lib/utils/text_markdown';
+import IndentHelper from './helpers/indent_helper';
+import { keystroke } from './lib/utils/common_utils';
+import * as keys from './lib/utils/keycodes';
+import UndoStack from './lib/utils/undo_stack';
 
 export default class GLForm {
   constructor(form, enableGFM = {}) {
+    this.handleKeyShortcuts = this.handleKeyShortcuts.bind(this);
+    this.setState = this.setState.bind(this);
+
     this.form = form;
     this.textarea = this.form.find('textarea.js-gfm-input');
     this.enableGFM = Object.assign({}, defaultAutocompleteConfig, enableGFM);
@@ -16,6 +23,10 @@ export default class GLForm {
         this.enableGFM[item] = Boolean(dataSources[item]);
       }
     });
+
+    this.undoStack = new UndoStack();
+    this.indentHelper = new IndentHelper(this.textarea[0]);
+
     // Before we start, we should clean up any previous data for this form
     this.destroy();
     // Set up the form
@@ -85,9 +96,84 @@ export default class GLForm {
   clearEventListeners() {
     this.textarea.off('focus');
     this.textarea.off('blur');
+    this.textarea.off('keydown');
     removeMarkdownListeners(this.form);
   }
 
+  setState(state) {
+    const selection = [this.textarea[0].selectionStart, this.textarea[0].selectionEnd];
+    this.textarea.val(state);
+    this.textarea[0].setSelectionRange(selection[0], selection[1]);
+  }
+
+  /*
+   Handle keypresses for a custom undo/redo stack.
+   We need this because the toolbar buttons and indentation helpers mess with the browser's
+   native undo/redo capability.
+   */
+  handleUndo(event) {
+    const content = this.textarea.val();
+    const { selectionStart, selectionEnd } = this.textarea[0];
+    const stack = this.undoStack;
+
+    if (stack.isEmpty()) {
+      // ==== Save initial state in undo history ====
+      stack.save(content);
+    }
+
+    if (keystroke(event, keys.Z_KEY_CODE, 'l')) {
+      // ==== Undo ====
+      event.preventDefault();
+      stack.save(content);
+      if (stack.canUndo()) {
+        this.setState(stack.undo());
+      }
+    } else if (keystroke(event, keys.Z_KEY_CODE, 'ls') || keystroke(event, keys.Y_KEY_CODE, 'l')) {
+      // ==== Redo ====
+      event.preventDefault();
+      if (stack.canRedo()) {
+        this.setState(stack.redo());
+      }
+    } else if (
+      keystroke(event, keys.SPACE_KEY_CODE) ||
+      keystroke(event, keys.ENTER_KEY_CODE) ||
+      selectionStart !== selectionEnd
+    ) {
+      // ==== Save after finishing a word or before deleting a large selection ====
+      stack.save(content);
+    } else if (content === '') {
+      // ==== Save after deleting everything ====
+      stack.save('');
+    } else {
+      // ==== Save after 1 second of inactivity ====
+      stack.scheduleSave(content);
+    }
+  }
+
+  handleIndent(event) {
+    if (keystroke(event, keys.LEFT_BRACKET_KEY_CODE, 'l')) {
+      // ==== Unindent selected lines ====
+      event.preventDefault();
+      this.indentHelper.unindent();
+    } else if (keystroke(event, keys.RIGHT_BRACKET_KEY_CODE, 'l')) {
+      // ==== Indent selected lines ====
+      event.preventDefault();
+      this.indentHelper.indent();
+    } else if (keystroke(event, keys.ENTER_KEY_CODE)) {
+      // ==== Auto-indent new lines ====
+      event.preventDefault();
+      this.indentHelper.newline();
+    } else if (keystroke(event, keys.BACKSPACE_KEY_CODE)) {
+      // ==== Auto-delete indents at the beginning of the line ====
+      this.indentHelper.backspace(event);
+    }
+  }
+
+  handleKeyShortcuts(event) {
+    this.handleIndent(event);
+    this.handleUndo(event);
+  }
+
   addEventListeners() {
     this.textarea.on('focus', function focusTextArea() {
       $(this)
@@ -99,5 +185,6 @@ export default class GLForm {
         .closest('.md-area')
         .removeClass('is-focused');
     });
+    this.textarea.on('keydown', e => this.handleKeyShortcuts(e.originalEvent));
   }
 }
diff --git a/app/assets/javascripts/helpers/indent_helper.js b/app/assets/javascripts/helpers/indent_helper.js
new file mode 100644
index 00000000000..a8815fac04e
--- /dev/null
+++ b/app/assets/javascripts/helpers/indent_helper.js
@@ -0,0 +1,182 @@
+const INDENT_SEQUENCE = '    ';
+
+function countLeftSpaces(text) {
+  const i = text.split('').findIndex(c => c !== ' ');
+  return i === -1 ? text.length : i;
+}
+
+/**
+ * IndentHelper provides methods that allow manual and smart indentation in
+ * textareas. It supports line indent/unindent, selection indent/unindent,
+ * auto indentation on newlines, and smart deletion of indents with backspace.
+ */
+export default class IndentHelper {
+  /**
+   * Creates a new IndentHelper and binds it to the given `textarea`. You can provide a custom indent sequence in the second parameter, but the `newline` and `backspace` operations may work funny if the indent sequence isn't spaces only.
+   */
+  constructor(textarea, indentSequence = INDENT_SEQUENCE) {
+    this.element = textarea;
+    this.seq = indentSequence;
+  }
+
+  getSelection() {
+    return { start: this.element.selectionStart, end: this.element.selectionEnd };
+  }
+
+  isRangeSelection() {
+    return this.element.selectionStart !== this.element.selectionEnd;
+  }
+
+  /**
+   * Re-implementation of textarea's setRangeText method, because IE/Edge don't support it.
+   *
+   * @see https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#dom-textarea%2Finput-setrangetext
+   */
+  setRangeText(replacement, start, end, selectMode) {
+    // Disable eslint to remain as faithful as possible to the above linked spec
+    /* eslint-disable no-param-reassign, no-case-declarations */
+    const text = this.element.value;
+
+    if (start > end) {
+      throw new RangeError('setRangeText: start index must be less than or equal to end index');
+    }
+
+    // Clamp to [0, len]
+    start = Math.max(0, Math.min(start, text.length));
+    end = Math.max(0, Math.min(end, text.length));
+
+    let selection = { start: this.element.selectionStart, end: this.element.selectionEnd };
+
+    this.element.value = text.slice(0, start) + replacement + text.slice(end);
+
+    const newLength = replacement.length;
+    const newEnd = start + newLength;
+
+    switch (selectMode) {
+      case 'select':
+        selection = { start, newEnd };
+        break;
+      case 'start':
+        selection = { start, end: start };
+        break;
+      case 'end':
+        selection = { start: newEnd, end: newEnd };
+        break;
+      case 'preserve':
+      default:
+        const oldLength = end - start;
+        const delta = newLength - oldLength;
+        if (selection.start > end) {
+          selection.start += delta;
+        } else if (selection.start > start) {
+          selection.start = start;
+        }
+        if (selection.end > end) {
+          selection.end += delta;
+        } else if (selection.end > start) {
+          selection.end = newEnd;
+        }
+    }
+
+    this.element.setSelectionRange(selection.start, selection.end);
+
+    /* eslint-enable no-param-reassign, no-case-declarations */
+  }
+
+  /**
+   * Returns an array of lines in the textarea, with information about their
+   * start/end offsets and whether they are included in the current selection.
+   */
+  splitLines() {
+    const { start, end } = this.getSelection();
+
+    const lines = this.element.value.split('\n');
+    let textStart = 0;
+    const lineObjects = [];
+    lines.forEach(line => {
+      const lineObj = {
+        text: line,
+        start: textStart,
+        end: textStart + line.length,
+      };
+      lineObj.inSelection = lineObj.start <= end && lineObj.end >= start;
+      lineObjects.push(lineObj);
+      textStart += line.length + 1;
+    });
+    return lineObjects;
+  }
+
+  /**
+   * Indents selected lines by one level.
+   */
+  indent() {
+    const { start } = this.getSelection();
+
+    const selectedLines = this.splitLines().filter(line => line.inSelection);
+    if (!this.isRangeSelection() && start === selectedLines[0].start) {
+      // Special case: if cursor is at the beginning of the line, move it one
+      // indent right.
+      const line = selectedLines[0];
+      this.setRangeText(this.seq, line.start, line.start, 'end');
+    } else {
+      selectedLines.reverse();
+      selectedLines.forEach(line => {
+        this.setRangeText(INDENT_SEQUENCE, line.start, line.start, 'preserve');
+      });
+    }
+  }
+
+  /**
+   * Unindents selected lines by one level.
+   */
+  unindent() {
+    const lines = this.splitLines().filter(line => line.inSelection);
+    lines.reverse();
+    lines
+      .filter(line => line.text.startsWith(this.seq))
+      .forEach(line => {
+        this.setRangeText('', line.start, line.start + this.seq.length, 'preserve');
+      });
+  }
+
+  /**
+   * Emulates a newline keypress, automatically indenting the new line.
+   */
+  newline() {
+    const { start, end } = this.getSelection();
+
+    if (this.isRangeSelection()) {
+      // Manually kill the selection before calculating the indent
+      this.setRangeText('', start, end, 'start');
+    }
+
+    // Auto-indent the next line
+    const currentLine = this.splitLines().find(line => line.end >= start);
+    const spaces = countLeftSpaces(currentLine.text);
+    this.setRangeText(`\n${' '.repeat(spaces)}`, start, start, 'end');
+  }
+
+  /**
+   * If the cursor is positioned at the end of a line's leading indents,
+   * emulates a backspace keypress by deleting a single level of indents.
+   * @param event The DOM KeyboardEvent that triggers this action, or null.
+   */
+  backspace(event) {
+    const { start } = this.getSelection();
+
+    // If the cursor is at the end of leading indents, delete an indent.
+    if (!this.isRangeSelection()) {
+      const currentLine = this.splitLines().find(line => line.end >= start);
+      const cursorPosition = start - currentLine.start;
+      if (countLeftSpaces(currentLine.text) === cursorPosition && cursorPosition > 0) {
+        if (event) event.preventDefault();
+
+        let spacesToDelete = cursorPosition % this.seq.length;
+        if (spacesToDelete === 0) {
+          spacesToDelete = this.seq.length;
+        }
+        this.setRangeText('', start - spacesToDelete, start, 'start');
+      }
+    }
+  }
+}
diff --git a/app/assets/javascripts/lib/utils/common_utils.js b/app/assets/javascripts/lib/utils/common_utils.js
index 5e90893b684..1a94aee2398 100644
--- a/app/assets/javascripts/lib/utils/common_utils.js
+++ b/app/assets/javascripts/lib/utils/common_utils.js
@@ -203,6 +203,71 @@ export const isMetaKey = e => e.metaKey || e.ctrlKey || e.altKey || e.shiftKey;
 // 3) Middle-click or Mouse Wheel Click (e.which is 2)
 export const isMetaClick = e => e.metaKey || e.ctrlKey || e.which === 2;
 
+export const getPlatformLeaderKey = () => {
+  // eslint-disable-next-line @gitlab/i18n/no-non-i18n-strings
+  if (navigator && navigator.platform && navigator.platform.startsWith('Mac')) {
+    return 'meta';
+  }
+  return 'ctrl';
+};
+
+export const getPlatformLeaderKeyHTML = () => {
+  if (getPlatformLeaderKey() === 'meta') {
+    return '&#8984;';
+  }
+  // eslint-disable-next-line @gitlab/i18n/no-non-i18n-strings
+  return 'Ctrl';
+};
+
+export const isPlatformLeaderKey = e => {
+  if (getPlatformLeaderKey() === 'meta') {
+    return Boolean(e.metaKey);
+  }
+  return Boolean(e.ctrlKey);
+};
+
+/**
+ * Tests if a KeyboardEvent corresponds exactly to a keystroke.
+ *
+ * This function avoids hacking around an old version of Mousetrap, which we ship at the moment. It should be removed after we upgrade to the newest Mousetrap. See:
+ * - https://gitlab.com/gitlab-org/gitlab-ce/issues/63182
+ * - https://gitlab.com/gitlab-org/gitlab-ce/issues/64246
+ *
+ * @example
+ *     // Matches the enter key with exactly zero modifiers
+ *     keystroke(event, 13)
+ *
+ * @example
+ *     // Matches Control-Shift-Z
+ *     keystroke(event, 90, 'cs')
+ *
+ * @param e The KeyboardEvent to test.
+ * @param keyCode The key code of the key to test. Why keycodes? IE/Edge don't support the more convenient `key` and `code` properties.
+ * @param modifiers A string of modifiers keys. Each modifier key is represented by one character. The set of pressed modifier keys must match the given string exactly. Available options are 'a' for Alt/Option, 'c' for Control, 'm' for Meta/Command, 's' for Shift, and 'l' for the leader key (Meta on MacOS and Control otherwise).
+ * @returns {boolean} True if the KeyboardEvent corresponds to the given keystroke.
+ */
+export const keystroke = (e, keyCode, modifiers = '') => {
+  if (!e || !keyCode) {
+    return false;
+  }
+
+  const leader = getPlatformLeaderKey();
+  const mods = modifiers.toLowerCase().replace('l', leader.charAt(0));
+
+  // Match depressed modifier keys
+  if (
+    e.altKey !== mods.includes('a') ||
+    e.ctrlKey !== mods.includes('c') ||
+    e.metaKey !== mods.includes('m') ||
+    e.shiftKey !== mods.includes('s')
+  ) {
+    return false;
+  }
+
+  // Match the depressed key
+  return keyCode === (e.keyCode || e.which);
+};
+
 export const contentTop = () => {
   const perfBar = $('#js-peek').outerHeight() || 0;
   const mrTabsHeight = $('.merge-request-tabs').outerHeight() || 0;
diff --git a/app/assets/javascripts/lib/utils/keycodes.js b/app/assets/javascripts/lib/utils/keycodes.js
index 5e0f9b612a2..e24fcf47d71 100644
--- a/app/assets/javascripts/lib/utils/keycodes.js
+++ b/app/assets/javascripts/lib/utils/keycodes.js
@@ -1,4 +1,10 @@
-export const UP_KEY_CODE = 38;
-export const DOWN_KEY_CODE = 40;
+export const BACKSPACE_KEY_CODE = 8;
 export const ENTER_KEY_CODE = 13;
 export const ESC_KEY_CODE = 27;
+export const SPACE_KEY_CODE = 32;
+export const UP_KEY_CODE = 38;
+export const DOWN_KEY_CODE = 40;
+export const Y_KEY_CODE = 89;
+export const Z_KEY_CODE = 90;
+export const LEFT_BRACKET_KEY_CODE = 219;
+export const RIGHT_BRACKET_KEY_CODE = 221;
diff --git a/app/assets/javascripts/lib/utils/undo_stack.js b/app/assets/javascripts/lib/utils/undo_stack.js
new file mode 100644
index 00000000000..6cfdc2a0a0f
--- /dev/null
+++ b/app/assets/javascripts/lib/utils/undo_stack.js
@@ -0,0 +1,105 @@
+/**
+ * UndoStack provides a custom implementation of an undo/redo engine. It was originally written for GitLab's Markdown editor (`gl_form.js`), whose rich text editing capabilities broke native browser undo/redo behaviour.
+ *
+ * UndoStack supports predictable undos/redos, debounced saves, maximum history length, and duplicate detection.
+ *
+ * Usage:
+ *   - `stack = new UndoStack();`
+ *   - Saves a state to the stack with `stack.save(state)`.
+ *   - Get the current state with `stack.current()`.
+ *   - Revert to the previous state with `stack.undo()`.
+ *   - Redo a previous undo with `stack.redo()`;
+ *   - Queue a future save with `stack.scheduleSave(state, delay)`. Useful for text editors.
+ *   - See the full undo history in `stack.history`.
+ */
+export default class UndoStack {
+  constructor(maxLength = 1000) {
+    this.clear();
+    this.maxLength = maxLength;
+
+    // If you're storing reference-types in the undo stack, you might want to
+    // reassign this property to some deep-equals function.
+    this.comparator = (a, b) => a === b;
+  }
+
+  current() {
+    if (this.cursor === -1) {
+      return undefined;
+    }
+    return this.history[this.cursor];
+  }
+
+  isEmpty() {
+    return this.history.length === 0;
+  }
+
+  clear() {
+    this.clearPending();
+    this.history = [];
+    this.cursor = -1;
+  }
+
+  save(state) {
+    this.clearPending();
+    if (this.comparator(state, this.current())) {
+      // Don't save state if it's the same as the current state
+      return;
+    }
+
+    this.history.length = this.cursor + 1;
+    this.history.push(state);
+    this.cursor += 1;
+
+    if (this.history.length > this.maxLength) {
+      this.history.shift();
+      this.cursor -= 1;
+    }
+  }
+
+  scheduleSave(state, delay = 1000) {
+    this.clearPending();
+    this.pendingState = state;
+    this.timeout = setTimeout(this.saveNow.bind(this), delay);
+  }
+
+  saveNow() {
+    // Persists scheduled saves immediately
+    this.save(this.pendingState);
+    this.clearPending();
+  }
+
+  clearPending() {
+    // Cancels any scheduled saves
+    if (this.timeout) {
+      clearTimeout(this.timeout);
+      delete this.timeout;
+      delete this.pendingState;
+    }
+  }
+
+  canUndo() {
+    return this.cursor > 0;
+  }
+
+  undo() {
+    this.clearPending();
+    if (!this.canUndo()) {
+      return undefined;
+    }
+    this.cursor -= 1;
+    return this.history[this.cursor];
+  }
+
+  canRedo() {
+    return this.cursor >= 0 && this.cursor < this.history.length - 1;
+  }
+
+  redo() {
+    this.clearPending();
+    if (!this.canRedo()) {
+      return undefined;
+    }
+    this.cursor += 1;
+    return this.history[this.cursor];
+  }
+}
diff --git a/app/assets/javascripts/vue_shared/components/markdown/toolbar.vue b/app/assets/javascripts/vue_shared/components/markdown/toolbar.vue
index 8ce5b615795..21c44b59520 100644
--- a/app/assets/javascripts/vue_shared/components/markdown/toolbar.vue
+++ b/app/assets/javascripts/vue_shared/components/markdown/toolbar.vue
@@ -1,5 +1,6 @@
 <script>
 import { GlLink } from '@gitlab/ui';
+import { s__, sprintf } from '~/locale';
 
 export default {
   components: {
@@ -22,8 +23,28 @@ export default {
     },
   },
   computed: {
-    hasQuickActionsDocsPath() {
-      return this.quickActionsDocsPath !== '';
+    toolbarHelpHtml() {
+      const mdLinkStart = `<a href="${this.markdownDocsPath}" target="_blank" rel="noopener noreferrer" tabindex="-1">`;
+      const actionsLinkStart = `<a href="${this.quickActionsDocsPath}" target="_blank" rel="noopener noreferrer" tabindex="-1">`;
+      const linkEnd = '</a>';
+
+      if (this.markdownDocsPath && !this.quickActionsDocsPath) {
+        return sprintf(
+          s__('Editor|%{mdLinkStart}Markdown is supported%{mdLinkEnd}'),
+          { mdLinkStart, mdLinkEnd: linkEnd },
+          false,
+        );
+      } else if (this.markdownDocsPath && this.quickActionsDocsPath) {
+        return sprintf(
+          s__(
+            'Editor|%{mdLinkStart}Markdown%{mdLinkEnd} and %{actionsLinkStart}quick actions%{actionsLinkEnd} are supported',
+          ),
+          { mdLinkStart, mdLinkEnd: linkEnd, actionsLinkStart, actionsLinkEnd: linkEnd },
+          false,
+        );
+      }
+
+      return null;
     },
   },
 };
@@ -32,21 +53,7 @@ export default {
 <template>
   <div class="comment-toolbar clearfix">
     <div class="toolbar-text">
-      <template v-if="!hasQuickActionsDocsPath && markdownDocsPath">
-        <gl-link :href="markdownDocsPath" target="_blank" tabindex="-1">{{
-          __('Markdown is supported')
-        }}</gl-link>
-      </template>
-      <template v-if="hasQuickActionsDocsPath && markdownDocsPath">
-        <gl-link :href="markdownDocsPath" target="_blank" tabindex="-1">{{
-          __('Markdown')
-        }}</gl-link>
-        and
-        <gl-link :href="quickActionsDocsPath" target="_blank" tabindex="-1">{{
-          __('quick actions')
-        }}</gl-link>
-        are supported
-      </template>
+      <span v-html="toolbarHelpHtml"></span>
     </div>
     <span v-if="canAttachFile" class="uploading-container">
       <span class="uploading-progress-container hide">
diff --git a/app/views/shared/notes/_hints.html.haml b/app/views/shared/notes/_hints.html.haml
index fae7d6526e8..72ede50dd8c 100644
--- a/app/views/shared/notes/_hints.html.haml
+++ b/app/views/shared/notes/_hints.html.haml
@@ -1,14 +1,13 @@
 - supports_quick_actions = local_assigns.fetch(:supports_quick_actions, false)
 .comment-toolbar.clearfix
   .toolbar-text
-    = link_to _('Markdown'), help_page_path('user/markdown'), target: '_blank', tabindex: -1
+    - md_link_start = '<a href="%{url}" target="_blank" rel="noopener noreferrer" tabindex="-1">'.html_safe % { url: help_page_path('user/markdown') }
+    - actions_link_start = '<a href="%{url}" target="_blank" rel="noopener noreferrer" tabindex="-1">'.html_safe % { url: help_page_path('user/project/quick_actions') }
+    - link_end = '</a>'.html_safe
     - if supports_quick_actions
-      and
-      = link_to _('quick actions'), help_page_path('user/project/quick_actions'), target: '_blank', tabindex: -1
-      are
+      = s_('Editor|%{mdLinkStart}Markdown%{mdLinkEnd} and %{actionsLinkStart}quick actions%{actionsLinkEnd} are supported').html_safe % { mdLinkStart: md_link_start, mdLinkEnd: link_end, actionsLinkStart: actions_link_start, actionsLinkEnd: link_end }
     - else
-      is
-    supported
+      = s_('Editor|%{mdLinkStart}Markdown is supported%{mdLinkEnd}').html_safe % { mdLinkStart: md_link_start, mdLinkEnd: link_end }
 
   %span.uploading-container
     %span.uploading-progress-container.hide
diff --git a/changelogs/unreleased/mh-editor-indents.yml b/changelogs/unreleased/mh-editor-indents.yml
new file mode 100644
index 00000000000..a282c0f505d
--- /dev/null
+++ b/changelogs/unreleased/mh-editor-indents.yml
@@ -0,0 +1,5 @@
+---
+title: Markdown editors now have indentation shortcuts and auto-indentation
+merge_request: 28914
+author:
+type: added
diff --git a/locale/gitlab.pot b/locale/gitlab.pot
index ed71a4b42d9..114d245b688 100644
--- a/locale/gitlab.pot
+++ b/locale/gitlab.pot
@@ -3941,6 +3941,12 @@ msgstr ""
 msgid "Edit public deploy key"
 msgstr ""
 
+msgid "Editor|%{mdLinkStart}Markdown is supported%{mdLinkEnd}"
+msgstr ""
+
+msgid "Editor|%{mdLinkStart}Markdown%{mdLinkEnd} and %{actionsLinkStart}quick actions%{actionsLinkEnd} are supported"
+msgstr ""
+
 msgid "Email"
 msgstr ""
 
@@ -6382,18 +6388,12 @@ msgstr ""
 msgid "Mark to do as done"
 msgstr ""
 
-msgid "Markdown"
-msgstr ""
-
 msgid "Markdown Help"
 msgstr ""
 
 msgid "Markdown enabled"
 msgstr ""
 
-msgid "Markdown is supported"
-msgstr ""
-
 msgid "Marks this issue as a duplicate of %{duplicate_reference}."
 msgstr ""
 
@@ -13314,9 +13314,6 @@ msgstr ""
 msgid "project avatar"
 msgstr ""
 
-msgid "quick actions"
-msgstr ""
-
 msgid "register"
 msgstr ""
 
diff --git a/package.json b/package.json
index 4264064c93d..dce20505fb0 100644
--- a/package.json
+++ b/package.json
@@ -99,7 +99,7 @@
     "mermaid": "^8.2.3",
     "monaco-editor": "^0.15.6",
     "monaco-editor-webpack-plugin": "^1.7.0",
-    "mousetrap": "^1.4.6",
+    "mousetrap": "1.4.6",
     "pdfjs-dist": "^2.0.943",
     "pikaday": "^1.6.1",
     "popper.js": "^1.14.7",
diff --git a/spec/features/projects/wiki/user_creates_wiki_page_spec.rb b/spec/features/projects/wiki/user_creates_wiki_page_spec.rb
index aac095bfa6b..80741ace5d6 100644
--- a/spec/features/projects/wiki/user_creates_wiki_page_spec.rb
+++ b/spec/features/projects/wiki/user_creates_wiki_page_spec.rb
@@ -132,9 +132,15 @@ describe "User creates wiki page" do
 
         fill_in(:wiki_content, with: ascii_content)
 
-        page.within(".wiki-form") do
-          click_button("Create page")
-        end
+        # This is the dumbest bug in the world:
+        # When the #wiki_content textarea is filled in, JS captures the `Enter` keydown event in order to do
+        # auto-indentation and manually inserts a newline. However, for whatever reason, when you try to click on the
+        # submit button in Capybara, it will not trigger the `click` event if a \n or \r character has been manually
+        # added to the textarea. It will, however, trigger ALL OTHER EVENTS, including `mouseover`/down/up, focus, and
+        # blur. Just not `click`. But only when you manually insert \n or \r - if you manually insert any other sequence
+        # then `click` is fired normally. And it's only Capybara. Browsers and JSDOM don't have this issue.
+        # So that's why the next line performs the click via JS.
+        page.execute_script("document.querySelector('.qa-create-page-button').click()")
 
         page.within ".md" do
           expect(page).to have_selector(".katex", count: 3).and have_content("2+2 is 4")
diff --git a/spec/frontend/helpers/indent_helper_spec.js b/spec/frontend/helpers/indent_helper_spec.js
new file mode 100644
index 00000000000..fca12f0d1ef
--- /dev/null
+++ b/spec/frontend/helpers/indent_helper_spec.js
@@ -0,0 +1,371 @@
+import IndentHelper from '~/helpers/indent_helper';
+
+function createMockTextarea() {
+  const el = document.createElement('textarea');
+  el.setCursor = pos => el.setSelectionRange(pos, pos);
+  el.setCursorToEnd = () => el.setCursor(el.value.length);
+  el.selection = () => [el.selectionStart, el.selectionEnd];
+  el.cursor = () => {
+    const [start, end] = el.selection();
+    return start === end ? start : undefined;
+  };
+  return el;
+}
+
+describe('indent_helper', () => {
+  let element;
+  let ih;
+
+  beforeEach(() => {
+    element = createMockTextarea();
+    ih = new IndentHelper(element);
+  });
+
+  describe('indents', () => {
+    describe('a single line', () => {
+      it('when on an empty line; and cursor follows', () => {
+        element.value = '';
+        ih.indent();
+        expect(element.value).toBe('    ');
+        expect(element.cursor()).toBe(4);
+        ih.indent();
+        expect(element.value).toBe('        ');
+        expect(element.cursor()).toBe(8);
+      });
+
+      it('when at the start of a line; and cursor stays at start', () => {
+        element.value = 'foobar';
+        element.setCursor(0);
+        ih.indent();
+        expect(element.value).toBe('    foobar');
+        expect(element.cursor()).toBe(4);
+      });
+
+      it('when the cursor is in the middle; and cursor follows', () => {
+        element.value = 'foobar';
+        element.setCursor(3);
+        ih.indent();
+        expect(element.value).toBe('    foobar');
+        expect(element.cursor()).toBe(7);
+      });
+    });
+
+    describe('several lines', () => {
+      it('when everything is selected; and everything remains selected', () => {
+        element.value = 'foo\nbar\nbaz';
+        element.setSelectionRange(0, 11);
+        ih.indent();
+        expect(element.value).toBe('    foo\n    bar\n    baz');
+        expect(element.selection()).toEqual([0, 23]);
+      });
+
+      it('when all lines are partially selected; and the selection adapts', () => {
+        element.value = 'foo\nbar\nbaz';
+        element.setSelectionRange(2, 9);
+        ih.indent();
+        expect(element.value).toBe('    foo\n    bar\n    baz');
+        expect(element.selection()).toEqual([6, 21]);
+      });
+
+      it('when some lines are entirely selected; and entire lines remain selected', () => {
+        element.value = 'foo\nbar\nbaz';
+        element.setSelectionRange(4, 11);
+        ih.indent();
+        expect(element.value).toBe('foo\n    bar\n    baz');
+        expect(element.selection()).toEqual([4, 19]);
+      });
+
+      it('when some lines are partially selected; and the selection adapts', () => {
+        element.value = 'foo\nbar\nbaz';
+        element.setSelectionRange(5, 9);
+        ih.indent();
+        expect(element.value).toBe('foo\n    bar\n    baz');
+        expect(element.selection()).toEqual([5 + 4, 9 + 2 * 4]);
+      });
+
+      it('having different indentation when some lines are entirely selected; and entire lines remain selected', () => {
+        element.value = '    foo\nbar\n    baz';
+        element.setSelectionRange(8, 19);
+        ih.indent();
+        expect(element.value).toBe('    foo\n    bar\n        baz');
+        expect(element.selection()).toEqual([8, 27]);
+      });
+
+      it('having different indentation when some lines are partially selected; and the selection adapts', () => {
+        element.value = '    foo\nbar\n    baz';
+        element.setSelectionRange(9, 14);
+        ih.indent();
+        expect(element.value).toBe('    foo\n    bar\n        baz');
+        expect(element.selection()).toEqual([13, 22]);
+      });
+    });
+  });
+
+  describe('unindents', () => {
+    describe('a single line', () => {
+      it('but does nothing if there is not indent', () => {
+        element.value = 'foobar';
+        element.setCursor(2);
+        ih.unindent();
+        expect(element.value).toBe('foobar');
+        expect(element.cursor()).toBe(2);
+      });
+
+      it('but does nothing if there is a partial indent', () => {
+        element.value = '  foobar';
+        element.setCursor(1);
+        ih.unindent();
+        expect(element.value).toBe('  foobar');
+        expect(element.cursor()).toBe(1);
+      });
+
+      it('when the cursor is in the line text; cursor follows', () => {
+        element.value = '    foobar';
+        element.setCursor(6);
+        ih.unindent();
+        expect(element.value).toBe('foobar');
+        expect(element.cursor()).toBe(2);
+      });
+
+      it('when the cursor is in the indent; and cursor goes to start', () => {
+        element.value = '    foobar';
+        element.setCursor(2);
+        ih.unindent();
+        expect(element.value).toBe('foobar');
+        expect(element.cursor()).toBe(0);
+      });
+
+      it('when the cursor is at line start; and cursor stays at start', () => {
+        element.value = '    foobar';
+        element.setCursor(0);
+        ih.unindent();
+        expect(element.value).toBe('foobar');
+        expect(element.cursor()).toBe(0);
+      });
+
+      it('when a selection includes part of the indent and text', () => {
+        element.value = '    foobar';
+        element.setSelectionRange(2, 8);
+        ih.unindent();
+        expect(element.value).toBe('foobar');
+        expect(element.selection()).toEqual([0, 4]);
+      });
+
+      it('when a selection includes part of the indent only', () => {
+        element.value = '    foobar';
+        element.setSelectionRange(0, 4);
+        ih.unindent();
+        expect(element.value).toBe('foobar');
+        expect(element.cursor()).toBe(0);
+
+        element.value = '    foobar';
+        element.setSelectionRange(1, 3);
+        ih.unindent();
+        expect(element.value).toBe('foobar');
+        expect(element.cursor()).toBe(0);
+      });
+    });
+
+    describe('several lines', () => {
+      it('when everything is selected', () => {
+        element.value = '    foo\n        bar\n    baz';
+        element.setSelectionRange(0, 27);
+        ih.unindent();
+        expect(element.value).toBe('foo\n    bar\nbaz');
+        expect(element.selection()).toEqual([0, 15]);
+      });
+
+      it('when all lines are partially selected', () => {
+        element.value = '    foo\n        bar\n    baz';
+        element.setSelectionRange(5, 26);
+        ih.unindent();
+        expect(element.value).toBe('foo\n    bar\nbaz');
+        expect(element.selection()).toEqual([1, 14]);
+      });
+
+      it('when all lines are entirely selected', () => {
+        element.value = '    foo\n        bar\n    baz';
+        element.setSelectionRange(8, 27);
+        ih.unindent();
+        expect(element.value).toBe('    foo\n    bar\nbaz');
+        expect(element.selection()).toEqual([8, 19]);
+      });
+
+      it('when some lines are entirely selected', () => {
+        element.value = '    foo\n        bar\n    baz';
+        element.setSelectionRange(8, 27);
+        ih.unindent();
+        expect(element.value).toBe('    foo\n    bar\nbaz');
+        expect(element.selection()).toEqual([8, 19]);
+      });
+
+      it('when some lines are partially selected', () => {
+        element.value = '    foo\n        bar\n    baz';
+        element.setSelectionRange(17, 26);
+        ih.unindent();
+        expect(element.value).toBe('    foo\n    bar\nbaz');
+        expect(element.selection()).toEqual([13, 18]);
+      });
+
+      it('when some lines are partially selected within their indents', () => {
+        element.value = '    foo\n        bar\n    baz';
+        element.setSelectionRange(10, 22);
+        ih.unindent();
+        expect(element.value).toBe('    foo\n    bar\nbaz');
+        expect(element.selection()).toEqual([8, 16]);
+      });
+    });
+  });
+
+  describe('newline', () => {
+    describe('on a single line', () => {
+      it('auto-indents the new line', () => {
+        element.value = 'foo\n bar\n    baz\n        qux';
+
+        element.setCursor(3);
+        ih.newline();
+        expect(element.value).toBe('foo\n\n bar\n    baz\n        qux');
+        expect(element.cursor()).toBe(4);
+
+        element.setCursor(9);
+        ih.newline();
+        expect(element.value).toBe('foo\n\n bar\n \n    baz\n        qux');
+        expect(element.cursor()).toBe(11);
+
+        element.setCursor(19);
+        ih.newline();
+        expect(element.value).toBe('foo\n\n bar\n \n    baz\n    \n        qux');
+        expect(element.cursor()).toBe(24);
+
+        element.setCursor(36);
+        ih.newline();
+        expect(element.value).toBe('foo\n\n bar\n \n    baz\n    \n        qux\n        ');
+        expect(element.cursor()).toBe(45);
+      });
+
+      it('splits a line and auto-indents', () => {
+        element.value = '    foobar';
+        element.setCursor(7);
+        ih.newline();
+        expect(element.value).toBe('    foo\n    bar');
+        expect(element.cursor()).toBe(12);
+      });
+
+      it('replaces selection with an indented newline', () => {
+        element.value = '    foobarbaz';
+        element.setSelectionRange(7, 10);
+        ih.newline();
+        expect(element.value).toBe('    foo\n    baz');
+        expect(element.cursor()).toBe(12);
+      });
+    });
+
+    it('on several lines.replaces selection with indented newline', () => {
+      element.value = '  foo\n    bar\n  baz';
+      element.setSelectionRange(4, 17);
+      ih.newline();
+      expect(element.value).toBe('  fo\n  az');
+      expect(element.cursor()).toBe(7);
+    });
+  });
+
+  describe('backspace', () => {
+    let event;
+
+    // This suite tests only the special indent-removing behaviour of the
+    // backspace() method, since non-special cases are handled natively as a
+    // backspace keypress.
+
+    beforeEach(() => {
+      event = { preventDefault: jest.fn() };
+    });
+
+    describe('on a single line', () => {
+      it('does nothing special if in the line text', () => {
+        element.value = '    foobar';
+        element.setCursor(7);
+        ih.backspace(event);
+        expect(event.preventDefault).not.toHaveBeenCalled();
+      });
+
+      it('does nothing special if after a non-leading indent', () => {
+        element.value = '    foo    bar';
+        element.setCursor(11);
+        ih.backspace(event);
+        expect(event.preventDefault).not.toHaveBeenCalled();
+      });
+
+      it('deletes one leading indent', () => {
+        element.value = '        foo';
+        element.setCursor(8);
+        ih.backspace(event);
+        expect(event.preventDefault).toHaveBeenCalled();
+        expect(element.value).toBe('    foo');
+        expect(element.cursor()).toBe(4);
+      });
+
+      it('does nothing if cursor is inside the leading indent', () => {
+        element.value = '        foo';
+        element.setCursor(4);
+        ih.backspace(event);
+        expect(event.preventDefault).not.toHaveBeenCalled();
+      });
+
+      it('does nothing if cursor is at the start of the line', () => {
+        element.value = '    foo';
+        element.setCursor(0);
+        ih.backspace(event);
+        expect(event.preventDefault).not.toHaveBeenCalled();
+      });
+
+      it('deletes one partial indent', () => {
+        element.value = '      foo';
+        element.setCursor(6);
+        ih.backspace(event);
+        expect(event.preventDefault).toHaveBeenCalled();
+        expect(element.value).toBe('    foo');
+        expect(element.cursor()).toBe(4);
+      });
+
+      it('deletes indents sequentially', () => {
+        element.value = '          foo';
+        element.setCursor(10);
+        ih.backspace(event);
+        ih.backspace(event);
+        ih.backspace(event);
+        expect(event.preventDefault).toHaveBeenCalled();
+        expect(element.value).toBe('foo');
+        expect(element.cursor()).toBe(0);
+      });
+    });
+
+    describe('on several lines', () => {
+      it('deletes indent only on its own line', () => {
+        element.value = '    foo\n        bar\n    baz';
+        element.setCursor(16);
+        ih.backspace(event);
+        expect(event.preventDefault).toHaveBeenCalled();
+        expect(element.value).toBe('    foo\n    bar\n    baz');
+        expect(element.cursor()).toBe(12);
+      });
+
+      it('has no special behaviour with any range selection', () => {
+        const text = '  foo\n    bar\n  baz';
+        for (let start = 0; start < text.length; start += 1) {
+          for (let end = start + 1; end < text.length; end += 1) {
+            element.value = text;
+            element.setSelectionRange(start, end);
+            ih.backspace(event);
+            expect(event.preventDefault).not.toHaveBeenCalled();
+
+            // Ensure that the backspace() method doesn't change state
+            // In reality, these two statements won't hold because the browser
+            // will natively process the backspace event.
+            expect(element.value).toBe(text);
+            expect(element.selection()).toEqual([start, end]);
+          }
+        }
+      });
+    });
+  });
+});
diff --git a/spec/frontend/lib/utils/common_utils_spec.js b/spec/frontend/lib/utils/common_utils_spec.js
new file mode 100644
index 00000000000..e3d3b82d2f3
--- /dev/null
+++ b/spec/frontend/lib/utils/common_utils_spec.js
@@ -0,0 +1,180 @@
+import * as cu from '~/lib/utils/common_utils';
+
+const CMD_ENTITY = '&#8984;';
+
+// Redefine `navigator.platform` because it's unsettable by default in JSDOM.
+let platform;
+Object.defineProperty(navigator, 'platform', {
+  configurable: true,
+  get: () => platform,
+  set: val => {
+    platform = val;
+  },
+});
+
+describe('common_utils', () => {
+  describe('platform leader key helpers', () => {
+    const CTRL_EVENT = { ctrlKey: true };
+    const META_EVENT = { metaKey: true };
+    const BOTH_EVENT = { ctrlKey: true, metaKey: true };
+
+    it('should return "ctrl" if navigator.platform is unset', () => {
+      expect(cu.getPlatformLeaderKey()).toBe('ctrl');
+      expect(cu.getPlatformLeaderKeyHTML()).toBe('Ctrl');
+      expect(cu.isPlatformLeaderKey(CTRL_EVENT)).toBe(true);
+      expect(cu.isPlatformLeaderKey(META_EVENT)).toBe(false);
+      expect(cu.isPlatformLeaderKey(BOTH_EVENT)).toBe(true);
+    });
+
+    it('should return "meta" on MacOS', () => {
+      navigator.platform = 'MacIntel';
+      expect(cu.getPlatformLeaderKey()).toBe('meta');
+      expect(cu.getPlatformLeaderKeyHTML()).toBe(CMD_ENTITY);
+      expect(cu.isPlatformLeaderKey(CTRL_EVENT)).toBe(false);
+      expect(cu.isPlatformLeaderKey(META_EVENT)).toBe(true);
+      expect(cu.isPlatformLeaderKey(BOTH_EVENT)).toBe(true);
+    });
+
+    it('should return "ctrl" on Linux', () => {
+      navigator.platform = 'Linux is great';
+      expect(cu.getPlatformLeaderKey()).toBe('ctrl');
+      expect(cu.getPlatformLeaderKeyHTML()).toBe('Ctrl');
+      expect(cu.isPlatformLeaderKey(CTRL_EVENT)).toBe(true);
+      expect(cu.isPlatformLeaderKey(META_EVENT)).toBe(false);
+      expect(cu.isPlatformLeaderKey(BOTH_EVENT)).toBe(true);
+    });
+
+    it('should return "ctrl" on Windows', () => {
+      navigator.platform = 'Win32';
+      expect(cu.getPlatformLeaderKey()).toBe('ctrl');
+      expect(cu.getPlatformLeaderKeyHTML()).toBe('Ctrl');
+      expect(cu.isPlatformLeaderKey(CTRL_EVENT)).toBe(true);
+      expect(cu.isPlatformLeaderKey(META_EVENT)).toBe(false);
+      expect(cu.isPlatformLeaderKey(BOTH_EVENT)).toBe(true);
+    });
+  });
+
+  describe('keystroke', () => {
+    const CODE_BACKSPACE = 8;
+    const CODE_TAB = 9;
+    const CODE_ENTER = 13;
+    const CODE_SPACE = 32;
+    const CODE_4 = 52;
+    const CODE_F = 70;
+    const CODE_Z = 90;
+
+    // Helper function that quickly creates KeyboardEvents
+    const k = (code, modifiers = '') => ({
+      keyCode: code,
+      which: code,
+      altKey: modifiers.includes('a'),
+      ctrlKey: modifiers.includes('c'),
+      metaKey: modifiers.includes('m'),
+      shiftKey: modifiers.includes('s'),
+    });
+
+    const EV_F = k(CODE_F);
+    const EV_ALT_F = k(CODE_F, 'a');
+    const EV_CONTROL_F = k(CODE_F, 'c');
+    const EV_META_F = k(CODE_F, 'm');
+    const EV_SHIFT_F = k(CODE_F, 's');
+    const EV_CONTROL_SHIFT_F = k(CODE_F, 'cs');
+    const EV_ALL_F = k(CODE_F, 'scma');
+    const EV_ENTER = k(CODE_ENTER);
+    const EV_TAB = k(CODE_TAB);
+    const EV_SPACE = k(CODE_SPACE);
+    const EV_BACKSPACE = k(CODE_BACKSPACE);
+    const EV_4 = k(CODE_4);
+    const EV_$ = k(CODE_4, 's');
+
+    const { keystroke } = cu;
+
+    it('short-circuits with bad arguments', () => {
+      expect(keystroke()).toBe(false);
+      expect(keystroke({})).toBe(false);
+    });
+
+    it('handles keystrokes using key codes', () => {
+      // Test a letter key with modifiers
+      expect(keystroke(EV_F, CODE_F)).toBe(true);
+      expect(keystroke(EV_F, CODE_F, '')).toBe(true);
+      expect(keystroke(EV_ALT_F, CODE_F, 'a')).toBe(true);
+      expect(keystroke(EV_CONTROL_F, CODE_F, 'c')).toBe(true);
+      expect(keystroke(EV_META_F, CODE_F, 'm')).toBe(true);
+      expect(keystroke(EV_SHIFT_F, CODE_F, 's')).toBe(true);
+      expect(keystroke(EV_CONTROL_SHIFT_F, CODE_F, 'cs')).toBe(true);
+      expect(keystroke(EV_ALL_F, CODE_F, 'acms')).toBe(true);
+
+      // Test non-letter keys
+      expect(keystroke(EV_TAB, CODE_TAB)).toBe(true);
+      expect(keystroke(EV_ENTER, CODE_ENTER)).toBe(true);
+      expect(keystroke(EV_SPACE, CODE_SPACE)).toBe(true);
+      expect(keystroke(EV_BACKSPACE, CODE_BACKSPACE)).toBe(true);
+
+      // Test a number/symbol key
+      expect(keystroke(EV_4, CODE_4)).toBe(true);
+      expect(keystroke(EV_$, CODE_4, 's')).toBe(true);
+
+      // Test wrong input
+      expect(keystroke(EV_F, CODE_Z)).toBe(false);
+      expect(keystroke(EV_SHIFT_F, CODE_F)).toBe(false);
+      expect(keystroke(EV_SHIFT_F, CODE_F, 'c')).toBe(false);
+    });
+
+    it('is case-insensitive', () => {
+      expect(keystroke(EV_ALL_F, CODE_F, 'ACMS')).toBe(true);
+    });
+
+    it('handles bogus inputs', () => {
+      expect(keystroke(EV_F, 'not a keystroke')).toBe(false);
+      expect(keystroke(EV_F, null)).toBe(false);
+    });
+
+    it('handles exact modifier keys, in any order', () => {
+      // Test permutations of modifiers
+      expect(keystroke(EV_ALL_F, CODE_F, 'acms')).toBe(true);
+      expect(keystroke(EV_ALL_F, CODE_F, 'smca')).toBe(true);
+      expect(keystroke(EV_ALL_F, CODE_F, 'csma')).toBe(true);
+      expect(keystroke(EV_CONTROL_SHIFT_F, CODE_F, 'cs')).toBe(true);
+      expect(keystroke(EV_CONTROL_SHIFT_F, CODE_F, 'sc')).toBe(true);
+
+      // Test wrong modifiers
+      expect(keystroke(EV_ALL_F, CODE_F, 'smca')).toBe(true);
+      expect(keystroke(EV_ALL_F, CODE_F)).toBe(false);
+      expect(keystroke(EV_ALL_F, CODE_F, '')).toBe(false);
+      expect(keystroke(EV_ALL_F, CODE_F, 'c')).toBe(false);
+      expect(keystroke(EV_ALL_F, CODE_F, 'ca')).toBe(false);
+      expect(keystroke(EV_ALL_F, CODE_F, 'ms')).toBe(false);
+      expect(keystroke(EV_CONTROL_SHIFT_F, CODE_F, 'cs')).toBe(true);
+      expect(keystroke(EV_CONTROL_SHIFT_F, CODE_F, 'c')).toBe(false);
+      expect(keystroke(EV_CONTROL_SHIFT_F, CODE_F, 's')).toBe(false);
+      expect(keystroke(EV_CONTROL_SHIFT_F, CODE_F, 'csa')).toBe(false);
+      expect(keystroke(EV_CONTROL_SHIFT_F, CODE_F, 'm')).toBe(false);
+      expect(keystroke(EV_SHIFT_F, CODE_F, 's')).toBe(true);
+      expect(keystroke(EV_SHIFT_F, CODE_F, 'c')).toBe(false);
+      expect(keystroke(EV_SHIFT_F, CODE_F, 'csm')).toBe(false);
+    });
+
+    it('handles the platform-dependent leader key', () => {
+      navigator.platform = 'Win32';
+      let EV_UNDO = k(CODE_Z, 'c');
+      let EV_REDO = k(CODE_Z, 'cs');
+      expect(keystroke(EV_UNDO, CODE_Z, 'l')).toBe(true);
+      expect(keystroke(EV_UNDO, CODE_Z, 'c')).toBe(true);
+      expect(keystroke(EV_UNDO, CODE_Z, 'm')).toBe(false);
+      expect(keystroke(EV_REDO, CODE_Z, 'sl')).toBe(true);
+      expect(keystroke(EV_REDO, CODE_Z, 'sc')).toBe(true);
+      expect(keystroke(EV_REDO, CODE_Z, 'sm')).toBe(false);
+
+      navigator.platform = 'MacIntel';
+      EV_UNDO = k(CODE_Z, 'm');
+      EV_REDO = k(CODE_Z, 'ms');
+      expect(keystroke(EV_UNDO, CODE_Z, 'l')).toBe(true);
+      expect(keystroke(EV_UNDO, CODE_Z, 'c')).toBe(false);
+      expect(keystroke(EV_UNDO, CODE_Z, 'm')).toBe(true);
+      expect(keystroke(EV_REDO, CODE_Z, 'sl')).toBe(true);
+      expect(keystroke(EV_REDO, CODE_Z, 'sc')).toBe(false);
+      expect(keystroke(EV_REDO, CODE_Z, 'sm')).toBe(true);
+    });
+  });
+});
diff --git a/spec/frontend/lib/utils/undo_stack_spec.js b/spec/frontend/lib/utils/undo_stack_spec.js
new file mode 100644
index 00000000000..31ad0e77d6f
--- /dev/null
+++ b/spec/frontend/lib/utils/undo_stack_spec.js
@@ -0,0 +1,237 @@
+import UndoStack from '~/lib/utils/undo_stack';
+
+import { isEqual } from 'underscore';
+
+describe('UndoStack', () => {
+  let stack;
+
+  beforeEach(() => {
+    stack = new UndoStack();
+  });
+
+  afterEach(() => {
+    // Make sure there's not pending saves
+    const history = Array.from(stack.history);
+    jest.runAllTimers();
+    expect(stack.history).toEqual(history);
+  });
+
+  it('is blank on construction', () => {
+    expect(stack.isEmpty()).toBe(true);
+    expect(stack.history).toEqual([]);
+    expect(stack.cursor).toBe(-1);
+    expect(stack.canUndo()).toBe(false);
+    expect(stack.canRedo()).toBe(false);
+  });
+
+  it('handles simple undo/redo behaviour', () => {
+    stack.save(10);
+    stack.save(11);
+    stack.save(12);
+
+    expect(stack.history).toEqual([10, 11, 12]);
+    expect(stack.cursor).toBe(2);
+    expect(stack.current()).toBe(12);
+    expect(stack.isEmpty()).toBe(false);
+    expect(stack.canUndo()).toBe(true);
+    expect(stack.canRedo()).toBe(false);
+
+    stack.undo();
+    expect(stack.history).toEqual([10, 11, 12]);
+    expect(stack.current()).toBe(11);
+    expect(stack.canUndo()).toBe(true);
+    expect(stack.canRedo()).toBe(true);
+
+    stack.undo();
+    expect(stack.current()).toBe(10);
+    expect(stack.canUndo()).toBe(false);
+    expect(stack.canRedo()).toBe(true);
+
+    stack.redo();
+    expect(stack.current()).toBe(11);
+
+    stack.redo();
+    expect(stack.current()).toBe(12);
+    expect(stack.isEmpty()).toBe(false);
+    expect(stack.canUndo()).toBe(true);
+    expect(stack.canRedo()).toBe(false);
+
+    // Saving should clear the redo stack
+    stack.undo();
+    stack.save(13);
+    expect(stack.history).toEqual([10, 11, 13]);
+    expect(stack.current()).toBe(13);
+  });
+
+  it('clear() should clear the undo history', () => {
+    stack.save(0);
+    stack.save(1);
+    stack.save(2);
+    stack.clear();
+    expect(stack.history).toEqual([]);
+    expect(stack.current()).toBeUndefined();
+  });
+
+  it('undo and redo are no-ops if unavailable', () => {
+    stack.save(10);
+    expect(stack.canRedo()).toBe(false);
+    expect(stack.canUndo()).toBe(false);
+
+    stack.save(11);
+    expect(stack.canRedo()).toBe(false);
+    expect(stack.canUndo()).toBe(true);
+
+    expect(stack.redo()).toBeUndefined();
+    expect(stack.history).toEqual([10, 11]);
+    expect(stack.current()).toBe(11);
+    expect(stack.canRedo()).toBe(false);
+    expect(stack.canUndo()).toBe(true);
+
+    expect(stack.undo()).toBe(10);
+    expect(stack.undo()).toBeUndefined();
+    expect(stack.history).toEqual([10, 11]);
+    expect(stack.current()).toBe(10);
+    expect(stack.canRedo()).toBe(true);
+    expect(stack.canUndo()).toBe(false);
+  });
+
+  it('should not save a duplicate state', () => {
+    stack.save(10);
+    stack.save(11);
+    stack.save(11);
+    stack.save(10);
+    stack.save(10);
+
+    expect(stack.history).toEqual([10, 11, 10]);
+  });
+
+  it('uses the === operator to detect duplicates', () => {
+    stack.save(10);
+    stack.save(10);
+    expect(stack.history).toEqual([10]);
+
+    // eslint-disable-next-line eqeqeq
+    expect(2 == '2' && '2' == 2).toBe(true);
+    stack.clear();
+    stack.save(2);
+    stack.save(2);
+    stack.save('2');
+    stack.save('2');
+    stack.save(2);
+    expect(stack.history).toEqual([2, '2', 2]);
+
+    const obj = {};
+    stack.clear();
+    stack.save(obj);
+    stack.save(obj);
+    stack.save({});
+    stack.save({});
+    expect(stack.history).toEqual([{}, {}, {}]);
+  });
+
+  it('should allow custom comparators', () => {
+    stack.comparator = isEqual;
+    const obj = {};
+    stack.clear();
+    stack.save(obj);
+    stack.save(obj);
+    stack.save({});
+    stack.save({});
+    expect(stack.history).toEqual([{}]);
+  });
+
+  it('should enforce a max number of undo states', () => {
+    // Try 2000 saves. Only the last 1000 should be preserved.
+    const sequence = Array(2000)
+      .fill(0)
+      .map((el, i) => i);
+    sequence.forEach(stack.save.bind(stack));
+    expect(stack.history.length).toBe(1000);
+    expect(stack.history).toEqual(sequence.slice(1000));
+    expect(stack.current()).toBe(1999);
+    expect(stack.canUndo()).toBe(true);
+    expect(stack.canRedo()).toBe(false);
+
+    // Saving drops the oldest elements from the stack
+    stack.save('end');
+    expect(stack.history.length).toBe(1000);
+    expect(stack.current()).toBe('end');
+    expect(stack.history).toEqual([...sequence.slice(1001), 'end']);
+
+    // If states were undone but the history is full, can still add.
+    stack.undo();
+    stack.undo();
+    expect(stack.current()).toBe(1998);
+    stack.save(3000);
+    expect(stack.history.length).toBe(999);
+    // should be [1001, 1002, ..., 1998, 3000]
+    expect(stack.history).toEqual([...sequence.slice(1001, 1999), 3000]);
+
+    // Try a different max length
+    stack = new UndoStack(2);
+    stack.save(0);
+    expect(stack.history).toEqual([0]);
+    stack.save(1);
+    expect(stack.history).toEqual([0, 1]);
+    stack.save(2);
+    expect(stack.history).toEqual([1, 2]);
+  });
+
+  describe('scheduled saves', () => {
+    it('should work', () => {
+      // Schedules 1000 ms ahead by default
+      stack.save(0);
+      stack.scheduleSave(1);
+      expect(stack.history).toEqual([0]);
+      jest.advanceTimersByTime(999);
+      expect(stack.history).toEqual([0]);
+      jest.advanceTimersByTime(1);
+      expect(stack.history).toEqual([0, 1]);
+    });
+
+    it('should have an adjustable delay', () => {
+      stack.scheduleSave(2, 100);
+      jest.advanceTimersByTime(100);
+      expect(stack.history).toEqual([2]);
+    });
+
+    it('should cancel previous scheduled saves', () => {
+      stack.scheduleSave(3);
+      jest.advanceTimersByTime(100);
+      stack.scheduleSave(4);
+      jest.runAllTimers();
+      expect(stack.history).toEqual([4]);
+    });
+
+    it('should be canceled by explicit saves', () => {
+      stack.scheduleSave(5);
+      stack.save(6);
+      jest.runAllTimers();
+      expect(stack.history).toEqual([6]);
+    });
+
+    it('should be canceled by undos and redos', () => {
+      stack.save(1);
+      stack.save(2);
+      stack.scheduleSave(3);
+      stack.undo();
+      jest.runAllTimers();
+      expect(stack.history).toEqual([1, 2]);
+      expect(stack.current()).toBe(1);
+
+      stack.scheduleSave(4);
+      stack.redo();
+      jest.runAllTimers();
+      expect(stack.history).toEqual([1, 2]);
+      expect(stack.current()).toBe(2);
+    });
+
+    it('should be persisted immediately with saveNow()', () => {
+      stack.scheduleSave(7);
+      stack.scheduleSave(8);
+      stack.saveNow();
+      jest.runAllTimers();
+      expect(stack.history).toEqual([8]);
+    });
+  });
+});
diff --git a/yarn.lock b/yarn.lock
index 4fa7665b000..0936e14c0a5 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -8340,7 +8340,7 @@ monaco-editor@^0.15.6:
   resolved "https://registry.yarnpkg.com/monaco-editor/-/monaco-editor-0.15.6.tgz#d63b3b06f86f803464f003b252627c3eb4a09483"
   integrity sha512-JoU9V9k6KqT9R9Tiw1RTU8ohZ+Xnf9DMg6Ktqqw5hILumwmq7xqa/KLXw513uTUsWbhtnHoSJYYR++u3pkyxJg==
 
-mousetrap@^1.4.6:
+mousetrap@1.4.6:
   version "1.4.6"
   resolved "https://registry.yarnpkg.com/mousetrap/-/mousetrap-1.4.6.tgz#eaca72e22e56d5b769b7555873b688c3332e390a"
   integrity sha1-6spy4i5W1bdpt1VYc7aIwzMuOQo=