Fix terminal combining marks

[MX48Live]

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Closes #46942

Problem

The terminal's monospace grid forcing logic in LineLayoutCache breaks rendering of combining marks in complex scripts. Thai, Lao, Arabic, Hebrew, Myanmar, Khmer, and other scripts use combining characters (vowels, tone marks, diacritics) that should render on top of or below their base character — not in a separate cell.

The root cause is in apply_force_width within line_layout.rs. After HarfBuzz correctly shapes the text (positioning combining marks at the same x coordinate as their base character), the force-width loop blindly increments the cell position counter for every glyph — including zero-advance combining marks. This displaces them into the next cell, breaking the visual output.

For example, Thai สวัสดี renders as ส ว ั ส ด ี with vowels and tone marks scattered across separate cells.

#39526 previously fixed Latin combining characters (like NFD-normalized é) by handling them at the terminal cell level via append_zero_width_chars. That fix works when alacritty's cell model already identifies the characters as zero-width. However, many complex script combining marks still get mispositioned at the shaping layer — which is what this PR addresses.

Solution

Extracted apply_force_width_to_layout() in line_layout.rs that detects combining marks during the force-width pass by checking whether a glyph's shaped x position has advanced by at least half a cell width beyond the previous base character. Combining marks (which HarfBuzz places at the same x as their base) are kept at their shaped position relative to the forced base, rather than being pushed to the next cell.

This approach is script-agnostic — it works for all complex scripts (Thai, Arabic, Devanagari, Myanmar, Khmer, etc.) without hardcoding Unicode ranges or disabling the monospace grid. The fix lives at the text shaping layer in GPUI, so it benefits any consumer of LineLayoutCache that uses force_width.

The two duplicate force-width loops (in layout_line and layout_line_by_hash) are consolidated into the single shared helper.

Testing

Unit tests cover the following scenarios:

• Latin text positions remain unchanged
• A single combining mark stays at its base character's cell
• A base character following combining marks gets the correct next cell
• Multiple stacked combining marks all stay anchored to cell 0
• Drifted base positions respect the existing 1px tolerance

Manual testing with the following scripts in the integrated terminal:

Script	Test text	What to look for
Thai	สวัสดีครับ ภาษาไทย ก้อนหิน น้ำตก	Vowel signs (ั ี) and tone marks (้) render above/below consonants, not in separate cells
Lao	ສະບາຍດີ ພາສາລາວ	Vowel signs stay attached to consonants
Arabic	بِسْمِ اللَّهِ الرَّحْمَٰنِ	Diacritics (harakat) render on their base letters
Hebrew	בְּרֵאשִׁית בָּרָא אֱלֹהִים	Vowel points (nikud) stay under/over their letters
Hindi	नमस्ते हिन्दी भाषा	Vowel matras render correctly on consonants
Bengali	বাংলা ভাষা নমস্কার	Combining marks stay attached
Tamil	தமிழ் மொழி வணக்கம்	Vowel signs render correctly
Myanmar	မြန်မာဘာသာ မင်္ဂလာပါ	Combining marks and medial consonants stay in place
Khmer	ភាសាខ្មែរ សួស្តី	Subscript consonants and vowel signs render correctly

Script to reproduce

echo "=== Thai ==="
echo "สวัสดีครับ ภาษาไทย ก้อนหิน น้ำตก"
echo ""
echo "=== Lao ==="
echo "ສະບາຍດີ ພາສາລາວ"
echo ""
echo "=== Arabic ==="
echo "بِسْمِ اللَّهِ الرَّحْمَٰنِ"
echo ""
echo "=== Hebrew ==="
echo "בְּרֵאשִׁית בָּרָא אֱלֹהִים"
echo ""
echo "=== Hindi (Devanagari) ==="
echo "नमस्ते हिन्दी भाषा"
echo ""
echo "=== Bengali ==="
echo "বাংলা ভাষা নমস্কার"
echo ""
echo "=== Tamil ==="
echo "தமிழ் மொழி வணக்கம்"
echo ""
echo "=== Myanmar ==="
echo "မြန်မာဘာသာ မင်္ဂလာပါ"
echo ""
echo "=== Khmer ==="
echo "ភាសាខ្មែរ សួស្តី"

Before

After

Tests passing

Release Notes:

• Fixed terminal rendering of combining marks in complex scripts (Thai, Arabic, Hebrew, Devanagari, Myanmar, Khmer, and others) where vowel signs and tone marks were incorrectly displaced to adjacent cells instead of rendering on their base characters.

[MX48Live]

Hi @as-cii @dinocosta 🙏

I've submitted this PR to fix the terminal combining marks issue (especially affecting Thai, Lao, Arabic, and other complex scripts where diacritics/vowels are rendered in wrong positions).

This is important for me ; w ; please consider

Thank you very much!

[ConradIrwin]

Interesting. I had expected we'd want to fix this using something like the unicode segmentation crate to do this, but this seems to work effectively and avoids drift between the text renderer and our hypotheticals.

Willing to give it a go. Thanks for the examples.

[staciax]

tysm 😭

[kittizz]

ขอบคุณครับ เฝ้ารอ release นี้อย่างตั้งใจ

[ConradIrwin]

ด้วยความยินดี!