From 401237f4f19690771b5383a08dd71a638f77b2a0 Mon Sep 17 00:00:00 2001
From: Matthias Clasen <mclasen@redhat.com>
Date: Thu, 11 Nov 2021 19:58:21 -0500
Subject: Try harder to not produce overlong lines

Our accounting for run lengths is imperfect (mainly
due to log widths for clusters being evenly distributed),
so it can happen that after reshaping the split item,
we find that it does not actually fit in the remaining
width. Previously, we would just use the split run
at that point and produce an overlong line. Instead,
undo the split, disable the breakpoint we used, and
try again.
---
 pango/pango-layout.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)

diff --git a/pango/pango-layout.c b/pango/pango-layout.c
index 7731532c..fad92b3b 100644
--- a/pango/pango-layout.c
+++ b/pango/pango-layout.c
@@ -3525,6 +3525,29 @@ shape_tab (PangoLayoutLine  *line,
     }
 }
 
+#define DISABLE_BREAKPOINT_FLAG (1 << 16)
+
+static inline void
+disable_breakpoint (PangoLayout *layout,
+                    int          offset)
+{
+  layout->log_attrs[offset].reserved |= DISABLE_BREAKPOINT_FLAG;
+}
+
+static inline gboolean
+breakpoint_is_disabled (PangoLayout *layout,
+                        int          offset)
+{
+  return (layout->log_attrs[offset].reserved & DISABLE_BREAKPOINT_FLAG) != 0;
+}
+
+static void
+clear_breakpoint_flags (PangoLayout *layout)
+{
+  for (int i = 0; i < layout->n_chars + 1; i++)
+    layout->log_attrs[i].reserved = 0;
+}
+
 static inline gboolean
 can_break_at (PangoLayout *layout,
               gint         offset,
@@ -3542,6 +3565,8 @@ can_break_at (PangoLayout *layout,
 
   if (offset == layout->n_chars)
     return TRUE;
+  else if (breakpoint_is_disabled (layout, offset))
+    return FALSE;
   else if (wrap == PANGO_WRAP_WORD)
     return layout->log_attrs[offset].is_line_break;
   else if (wrap == PANGO_WRAP_CHAR)
@@ -3990,7 +4015,28 @@ process_item (PangoLayout     *layout,
               /* Add the width back, to the line, reshape, subtract the new width */
               state->remaining_width += break_width;
               insert_run (line, state, new_item, FALSE);
+
               break_width = pango_glyph_string_get_width (((PangoGlyphItem *)(line->runs->data))->glyphs);
+              if (break_width > state->remaining_width &&
+                  !breakpoint_is_disabled (layout, break_num_chars) &&
+                  break_num_chars > 1)
+                {
+                  /* Unsplit the item, disable the breakpoint, try again */
+
+                  uninsert_run (line);
+                  pango_item_free (new_item);
+                  pango_item_unsplit (item, length, break_num_chars);
+
+                  disable_breakpoint (layout, break_num_chars);
+
+                  num_chars = item->num_chars;
+                  width = orig_width;
+                  break_num_chars = num_chars;
+                  break_width = width;
+
+                  goto retry_break;
+                }
+
               state->remaining_width -= break_width;
 
               state->log_widths_offset += break_num_chars;
@@ -4598,6 +4644,8 @@ pango_layout_check_lines (PangoLayout *layout)
     }
   while (!done);
 
+  clear_breakpoint_flags (layout);
+
   g_free (state.log_widths);
   g_list_free_full (state.baseline_shifts, g_free);
 
-- 
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