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MX48@LiveandConrad Irwin b5827ede3a Fix terminal combining marks (#53176 )

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] 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 |

<details>
<summary>Script to reproduce</summary>

```sh
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"
```

</details>

### Before

<img width="1144" height="740" alt="Screenshot 2569-04-05 at 10 43 00"
src="https://github.com/user-attachments/assets/bce2075a-9e19-40dd-81ea-0e29a451b781"
/>

### After

<img width="1144" height="747" alt="Screenshot 2569-04-05 at 11 22 55"
src="https://github.com/user-attachments/assets/e0613bf6-8452-4c65-b893-f9931a471b2e"
/>

### Tests passing

<img width="1144" height="740" alt="Screenshot 2569-04-05 at 11 20 22"
src="https://github.com/user-attachments/assets/9100bd77-4ea6-4f77-ae31-5129484552fd"
/>

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.

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

2026-04-23 02:50:58 +00:00

docs

docs: Change render function's return type (#27229 )

2025-03-20 22:48:22 -06:00

examples

gpui: Improve Anchored to support center position (#47154 )

2026-04-21 09:01:42 +00:00

resources/windows

Enable segment heap for Zed (#54538 )

2026-04-22 16:58:55 -04:00

src

Fix terminal combining marks (#53176 )

2026-04-23 02:50:58 +00:00

tests

Use serde 1.0.221 instead of serde_derive hackery (#38137 )

2025-09-14 14:01:04 +02:00

build.rs

GPUI on the web (#50228 )

2026-02-26 18:36:50 +01:00

Cargo.toml

gpui: Record input-to-frame latency histogram (#53822 )

2026-04-16 17:38:58 +00:00

LICENSE-APACHE

chore: Add crate licenses. (#4158 )

2024-01-23 16:56:22 +01:00

README.md

Revert "Fix link for Ownership and data flow in GPUI's README.md" (#47630 )

2026-01-26 08:30:33 +00:00

README.md

Welcome to GPUI!

GPUI is a hybrid immediate and retained mode, GPU accelerated, UI framework for Rust, designed to support a wide variety of applications.

Getting Started

GPUI is still in active development as we work on the Zed code editor, and is still pre-1.0. There will often be breaking changes between versions. You'll also need to use the latest version of stable Rust and be on macOS or Linux. Add the following to your Cargo.toml:

gpui = { version = "*" }

Ownership and data flow

Everything in GPUI starts with an Application. You can create one with Application::new(), and kick off your application by passing a callback to Application::run(). Inside this callback, you can create a new window with App::open_window(), and register your first root view. See gpui.rs for a complete example.

Dependencies

GPUI has various system dependencies that it needs in order to work.

macOS

On macOS, GPUI uses Metal for rendering. In order to use Metal, you need to do the following:

Install Xcode from the macOS App Store, or from the Apple Developer website. Note this requires a developer account.

Ensure you launch Xcode after installing, and install the macOS components, which is the default option.

Install Xcode command line tools
```
xcode-select --install
```
Ensure that the Xcode command line tools are using your newly installed copy of Xcode:
```
sudo xcode-select --switch /Applications/Xcode.app/Contents/Developer
```

The Big Picture

GPUI offers three different registers depending on your needs:

State management and communication with Entity's. Whenever you need to store application state that communicates between different parts of your application, you'll want to use GPUI's entities. Entities are owned by GPUI and are only accessible through an owned smart pointer similar to an Rc. See the app::context module for more information.
High level, declarative UI with views. All UI in GPUI starts with a view. A view is simply an Entity that can be rendered, by implementing the Render trait. At the start of each frame, GPUI will call this render method on the root view of a given window. Views build a tree of elements, lay them out and style them with a tailwind-style API, and then give them to GPUI to turn into pixels. See the div element for an all purpose swiss-army knife of rendering.
Low level, imperative UI with Elements. Elements are the building blocks of UI in GPUI, and they provide a nice wrapper around an imperative API that provides as much flexibility and control as you need. Elements have total control over how they and their child elements are rendered and can be used for making efficient views into large lists, implement custom layouting for a code editor, and anything else you can think of. See the element module for more information.

Each of these registers has one or more corresponding contexts that can be accessed from all GPUI services. This context is your main interface to GPUI, and is used extensively throughout the framework.

Other Resources

In addition to the systems above, GPUI provides a range of smaller services that are useful for building complex applications:

Actions are user-defined structs that are used for converting keystrokes into logical operations in your UI. Use this for implementing keyboard shortcuts, such as cmd-q. See the action module for more information.
Platform services, such as quit the app or open a URL are available as methods on the app::App.
An async executor that is integrated with the platform's event loop. See the executor module for more information.,
The [gpui::test] macro provides a convenient way to write tests for your GPUI applications. Tests also have their own kind of context, a TestAppContext which provides ways of simulating common platform input. See app::test_context and test modules for more details.

Currently, the best way to learn about these APIs is to read the Zed source code or drop a question in the Zed Discord. We're working on improving the documentation, creating more examples, and will be publishing more guides to GPUI on our blog.