Fix byte/character offset confusion in FlAccessibleTextField by robert-ancell · Pull Request #188138 · flutter/flutter

robert-ancell · 2026-06-17T23:59:50Z

get_line_at_offset and get_paragraph_at_offset used PangoLayoutLine start_index and length (byte offsets) directly as character offsets. They were compared against the ATK character offset, reported back as character offsets, and passed to get_substring (which uses g_utf8_substring, expecting character offsets).

For ASCII text byte and character offsets coincide, but for multi-byte UTF-8 text they diverge, producing incorrect substrings and offsets.

Convert byte indices to character offsets with g_utf8_pointer_to_offset before comparing, reporting, or substringing.

Add TextBoundaryMultiByte test covering line and paragraph boundaries on text containing multi-byte UTF-8 characters.

get_line_at_offset and get_paragraph_at_offset used PangoLayoutLine start_index and length (byte offsets) directly as character offsets. They were compared against the ATK character offset, reported back as character offsets, and passed to get_substring (which uses g_utf8_substring, expecting character offsets). For ASCII text byte and character offsets coincide, but for multi-byte UTF-8 text they diverge, producing incorrect substrings and offsets. Convert byte indices to character offsets with g_utf8_pointer_to_offset before comparing, reporting, or substringing. Add TextBoundaryMultiByte test covering line and paragraph boundaries on text containing multi-byte UTF-8 characters.

gemini-code-assist

Code Review

This pull request updates FlAccessibleTextField to compute line and paragraph boundaries using character offsets instead of byte offsets to correctly handle multi-byte UTF-8 characters, and adds a test case for this behavior. The review feedback suggests optimizing the performance of these computations by converting the input character offset to a byte offset once at the start of the functions, avoiding repeated $O(N)$ UTF-8 string traversals inside the loops.

mattkae

Looks to be an improvement overall! Just have a suggestion

mattkae · 2026-06-18T14:37:02Z

+    gint line_start = g_utf8_pointer_to_offset(text, text + line->start_index);
+    gint line_end =
+        g_utf8_pointer_to_offset(text, text + line->start_index + line->length);
+    if (offset >= line_start && offset <= line_end) {


Can we add a comment explaining how we'd get here?

Added some comments on the functions. The functions are still pretty complex but hopefully this helps.

Iterate the Pango layout lines with a for loop instead of a while loop, keeping the list iterator scoped to the loop. No functional change.

Add comments describing what each function returns, including the start and end character offsets and how lines and paragraphs are delimited.

mattkae

I am happy!

flutter/flutter@e228771...87224e0 2026-06-23 [email protected] Roll Dart SDK from 5cae7f9ada62 to 3a66ea7b9aaa (1 revision) (flutter/flutter#188379) 2026-06-23 [email protected] Roll Dart SDK from 1e6c246bb73a to 5cae7f9ada62 (2 revisions) (flutter/flutter#188370) 2026-06-23 [email protected] Roll Skia from 766f21ae61dc to ffac3e91fbc7 (24 revisions) (flutter/flutter#188366) 2026-06-23 [email protected] [Windows] Add public API to post task to platform thread (flutter/flutter#187365) 2026-06-23 [email protected] Roll Dart SDK from 7ab0179ce4d4 to 1e6c246bb73a (1 revision) (flutter/flutter#188354) 2026-06-23 [email protected] Fix byte/character offset confusion in FlAccessibleTextField (flutter/flutter#188138) 2026-06-22 [email protected] Roll Fuchsia Linux SDK from Lm76V7lvxVA0r1De5... to RymJjIj7dd5vQ3Cnh... (flutter/flutter#188353) 2026-06-22 [email protected] Roll pub packages (flutter/flutter#188355) 2026-06-22 [email protected] Sync CHANGELOG.md from stable (flutter/flutter#188331) 2026-06-22 [email protected] Roll Skia from 5fbb9bbd889c to 766f21ae61dc (2 revisions) (flutter/flutter#188184) 2026-06-22 49699333+dependabot[bot]@users.noreply.github.com Bump actions/checkout from 6.0.3 to 7.0.0 in the all-github-actions group (flutter/flutter#188350) 2026-06-22 [email protected] Use g_signal_connect_object in the Linux embedder (flutter/flutter#188241) 2026-06-22 [email protected] Disconnect from parent window signal when view is destroyed (flutter/flutter#185521) 2026-06-22 [email protected] Remove many absolute paths from build commands. (flutter/flutter#187765) 2026-06-22 [email protected] Fix spelling mistake in documentation (wether → whether) (flutter/flutter#186141) 2026-06-22 [email protected] Roll Dart SDK from a748c4b15399 to 7ab0179ce4d4 (2 revisions) (flutter/flutter#188332) 2026-06-22 [email protected] [Linux] Move compositor shader into its own GObject (flutter/flutter#188144) 2026-06-22 [email protected] Add agent skills for orchestrating cherry-picks to stable and beta channels (flutter/flutter#187860) 2026-06-22 [email protected] Roll Packages from c516c92 to cd5194a (1 revision) (flutter/flutter#188312) If this roll has caused a breakage, revert this CL and stop the roller using the controls here: https://autoroll.skia.org/r/flutter-packages Please CC [email protected] on the revert to ensure that a human is aware of the problem. To file a bug in Packages: https://github.com/flutter/flutter/issues/new/choose To report a problem with the AutoRoller itself, please file a bug: https://issues.skia.org/issues/new?component=1389291&template=1850622 Documentation for the AutoRoller is here: https://skia.googlesource.com/buildbot/+doc/main/autoroll/README.md

…#188138) get_line_at_offset and get_paragraph_at_offset used PangoLayoutLine start_index and length (byte offsets) directly as character offsets. They were compared against the ATK character offset, reported back as character offsets, and passed to get_substring (which uses g_utf8_substring, expecting character offsets). For ASCII text byte and character offsets coincide, but for multi-byte UTF-8 text they diverge, producing incorrect substrings and offsets. Convert byte indices to character offsets with g_utf8_pointer_to_offset before comparing, reporting, or substringing. Add TextBoundaryMultiByte test covering line and paragraph boundaries on text containing multi-byte UTF-8 characters.

robert-ancell requested a review from a team as a code owner June 17, 2026 23:59

flutter-dashboard Bot added the CICD Run CI/CD label Jun 17, 2026

github-actions Bot added a: text input Entering text in a text field or keyboard related problems engine flutter/engine related. See also e: labels. platform-linux Building on or for Linux specifically a: desktop Running on desktop team-linux Owned by the Linux platform team labels Jun 18, 2026

gemini-code-assist Bot reviewed Jun 18, 2026

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Comment thread engine/src/flutter/shell/platform/linux/fl_accessible_text_field.cc

Comment thread engine/src/flutter/shell/platform/linux/fl_accessible_text_field.cc

Comment thread engine/src/flutter/shell/platform/linux/fl_accessible_text_field.cc

mattkae previously approved these changes Jun 18, 2026

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robert-ancell added 2 commits June 19, 2026 12:17

Use a for loop in get_line_at_offset

b943499

Iterate the Pango layout lines with a for loop instead of a while loop, keeping the list iterator scoped to the loop. No functional change.

Document get_line_at_offset and get_paragraph_at_offset

104fe75

Add comments describing what each function returns, including the start and end character offsets and how lines and paragraphs are delimited.

robert-ancell dismissed mattkae’s stale review via 104fe75 June 22, 2026 00:16

github-actions Bot removed the CICD Run CI/CD label Jun 22, 2026

robert-ancell requested a review from mattkae June 22, 2026 00:20

mattkae previously approved these changes Jun 22, 2026

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Merge branch 'main' into fix-accessible-text-field-utf8-offsets

f0fb3a9

robert-ancell dismissed mattkae’s stale review via f0fb3a9 June 22, 2026 21:23

flutter-dashboard Bot added the CICD Run CI/CD label Jun 22, 2026

mattkae approved these changes Jun 22, 2026

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robert-ancell enabled auto-merge June 22, 2026 22:40

robert-ancell added this pull request to the merge queue Jun 23, 2026

Merged via the queue into flutter:master with commit 2594d16 Jun 23, 2026
201 of 202 checks passed

robert-ancell deleted the fix-accessible-text-field-utf8-offsets branch June 23, 2026 01:32

engine-flutter-autoroll mentioned this pull request Jun 23, 2026

Roll Flutter from e228771afe93 to 87224e0fb692 (19 revisions) flutter/packages#11961

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Fix byte/character offset confusion in FlAccessibleTextField#188138

Fix byte/character offset confusion in FlAccessibleTextField#188138
robert-ancell merged 4 commits into
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robert-ancell:fix-accessible-text-field-utf8-offsets

robert-ancell commented Jun 17, 2026

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