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LibWeb: Ensure hit testing is grapheme aware

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Previously, clicking in the middle of a multi-code point grapheme would
place the cursor at a code unit index somewhere in the middle of the
grapheme. This was not only visually misleading, but the user could then
start typing and insert characters in the middle of the cluster. This
also made text select pretty wonky.

The main issue was that we were treating the glyph index in a glyph run
as a code unit index. We must instead map that glyph index back to a
code unit index with help from LibGfx (via harfbuzz).

The distance computation used here was also a bit off, especially for
the last glyph in a glyph run. We essentially want the cursor to end
up on whichever edge of the clicked glyph it is closest to. The result
of the distance computation limited us to the left edge of the last
glyph. Instead, we can use the same edge tracking we use for form-
associated elements to handle this for us.
This commit is contained in:
Timothy Flynn 2025-08-19 13:35:13 -04:00 • committed by Andreas Kling
commit 1f88e6819a
Notes: github-actions[bot] 2025-08-22 12:07:55 +00:00
3 changed files with 36 additions and 9 deletions
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15
Libraries/LibWeb/Painting/PaintableFragment.cpp
View file

@ -5,6 +5,7 @@
*/
#include <LibWeb/DOM/Range.h>
#include <LibWeb/GraphemeEdgeTracker.h>
#include <LibWeb/HTML/FormAssociatedElement.h>
#include <LibWeb/HTML/HTMLInputElement.h>
#include <LibWeb/HTML/HTMLTextAreaElement.h>
@ -52,18 +53,14 @@ size_t PaintableFragment::index_in_node_for_point(CSSPixelPoint position) const
if (relative_inline_offset < 0)
return 0;
auto const& glyphs = m_glyph_run->glyphs();
auto smallest_distance = AK::NumericLimits<float>::max();
for (size_t i = 0; i < glyphs.size(); ++i) {
auto distance_to_position = AK::abs(glyphs[i].position.x() - relative_inline_offset);
GraphemeEdgeTracker tracker { relative_inline_offset };
// The last distance was smaller than this new distance, so we've found the closest glyph.
if (distance_to_position > smallest_distance)
return m_start_offset + i - 1;
smallest_distance = distance_to_position;
for (auto const& glyph : m_glyph_run->glyphs()) {
if (tracker.update(glyph.length_in_code_units, glyph.glyph_width) == IterationDecision::Break)
break;
}
return m_start_offset + m_length_in_code_units - 1;
return m_start_offset + tracker.resolve();
}
CSSPixelRect PaintableFragment::range_rect(Paintable::SelectionState selection_state, size_t start_offset_in_code_units, size_t end_offset_in_code_units) const

6
Tests/LibWeb/Text/expected/hit_testing/grapheme.txt Normal file
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@ -0,0 +1,6 @@
Click [13, 20]: position=0 text="hello 👩🏼‍❤️‍👨🏻👩🏼‍❤️‍👨🏻👩🏼‍❤️‍👨🏻👩🏼‍❤️‍👨🏻"
Click [16, 20]: position=1 text="ello 👩🏼‍❤️‍👨🏻👩🏼‍❤️‍👨🏻👩🏼‍❤️‍👨🏻👩🏼‍❤️‍👨🏻"
Click [52, 20]: position=6 text="👩🏼‍❤️‍👨🏻👩🏼‍❤️‍👨🏻👩🏼‍❤️‍👨🏻👩🏼‍❤️‍👨🏻"
Click [57, 20]: position=18 text="👩🏼‍❤️‍👨🏻👩🏼‍❤️‍👨🏻👩🏼‍❤️‍👨🏻"
Click [85, 20]: position=30 text="👩🏼‍❤️‍👨🏻👩🏼‍❤️‍👨🏻"
Click [90, 20]: position=42 text="👩🏼‍❤️‍👨🏻"

24
Tests/LibWeb/Text/input/hit_testing/grapheme.html Normal file
View file

@ -0,0 +1,24 @@
<!doctype html>
<script src="../include.js"></script>
<textarea id="text">
hello 👩🏼‍❤️‍👨🏻👩🏼‍❤️‍👨🏻👩🏼‍❤️‍👨🏻👩🏼‍❤️‍👨🏻
</textarea>
<script>
test(() => {
const content = text.textContent.trim();
const click = (x, y) => {
internals.click(x, y);
const remainingText = content.substr(text.selectionStart);
println(`Click [${x}, ${y}]: position=${text.selectionStart} text="${remainingText}"`);
};
click(13, 20);
click(16, 20);
click(52, 20);
click(57, 20);
click(85, 20);
click(90, 20);
});
</script>