1 files changed, 76 insertions, 0 deletions
diff --git a/app/assets/javascripts/diffs/utils/discussions.js b/app/assets/javascripts/diffs/utils/discussions.js
new file mode 100644
index 00000000000..c404705d209
--- /dev/null
+++ b/app/assets/javascripts/diffs/utils/discussions.js
@@ -0,0 +1,76 @@
+function normalize(processable) {
+  const { entry } = processable;
+  const offset = entry.rootBounds.bottom - entry.boundingClientRect.top;
+  const direction =
+    offset < 0 ? 'Up' : 'Down'; /* eslint-disable-line @gitlab/require-i18n-strings */
+
+  return {
+    ...processable,
+    entry: {
+      time: entry.time,
+      type: entry.isIntersecting ? 'intersection' : `scroll${direction}`,
+    },
+  };
+}
+
+function sort({ entry: alpha }, { entry: beta }) {
+  const diff = alpha.time - beta.time;
+  let order = 0;
+
+  if (diff < 0) {
+    order = -1;
+  } else if (diff > 0) {
+    order = 1;
+  } else if (alpha.type === 'intersection' && beta.type === 'scrollUp') {
+    order = 2;
+  } else if (alpha.type === 'scrollUp' && beta.type === 'intersection') {
+    order = -2;
+  }
+
+  return order;
+}
+
+function filter(entry) {
+  return entry.type !== 'scrollDown';
+}
+
+export function discussionIntersectionObserverHandlerFactory() {
+  let unprocessed = [];
+  let timer = null;
+
+  return (processable) => {
+    unprocessed.push(processable);
+
+    if (timer) {
+      clearTimeout(timer);
+    }
+
+    timer = setTimeout(() => {
+      unprocessed
+        .map(normalize)
+        .filter(filter)
+        .sort(sort)
+        .forEach((discussionObservationContainer) => {
+          const {
+            entry: { type },
+            currentDiscussion,
+            isFirstUnresolved,
+            isDiffsPage,
+            functions: { setCurrentDiscussionId, getPreviousUnresolvedDiscussionId },
+          } = discussionObservationContainer;
+
+          if (type === 'intersection') {
+            setCurrentDiscussionId(currentDiscussion.id);
+          } else if (type === 'scrollUp') {
+            setCurrentDiscussionId(
+              isFirstUnresolved
+                ? null
+                : getPreviousUnresolvedDiscussionId(currentDiscussion.id, isDiffsPage),
+            );
+          }
+        });
+
+      unprocessed = [];
+    }, 0);
+  };
+}