# gpui: Add scale-factor simulation to \`TestWindow\` \(\#63719\) · gitcafe/zed

[View on GitCafe](https://git.cafe/gitcafe/zed/commit/b95b188b59454bc2b01f08a54ab0182c4a55f141)

Repository: [gitcafe/zed](https://git.cafe/gitcafe/zed)

Visibility: public

Requested revision: b95b188b59454bc2b01f08a54ab0182c4a55f141

Requested commit: b95b188b59454bc2b01f08a54ab0182c4a55f141

Commit: b95b188b59454bc2b01f08a54ab0182c4a55f141

Tree: 5612da143c56be882f81b521e7876a63d0a21c31

Author: goring

Committer: GitHub

## Message

````
gpui: Add scale-factor simulation to `TestWindow` (#63719)

> **AI disclosure:** I used Claude (Anthropic) to write this change, its
test, and draft this PR description, after my own app shipped a
rendering bug that only reproduces on 1x displays — every test built on
gpui's test platform renders at a fixed 2.0 scale factor, so the bug was
invisible to an otherwise thorough offscreen test suite. I reviewed the
diff, ran the tests myself, and understand and take responsibility for
the change, per Zed's [AI
Policy](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#ai-policy).

# Objective

- `TestWindow::scale_factor()` has returned a hardcoded `2.0` since the
test platform's inception (`f5ba22659`, 2024-01-03), with no way for a
test to change it. Rendering behavior that only appears on 1x displays —
device-grid rounding, pixel snapping — can't be exercised: an error of
one device pixel is the full width of a hairline at 1x and half a
logical pixel at 2x, so this class of bug passes every test and ships.
- `Window::set_scale_factor` (`547a6cd59`, #61348, 2025-08) exists but
doesn't cover this: it overrides only the `Window`'s cached copy, so the
next `bounds_changed` re-reads the platform window and silently reverts
the override to 2.0.

## Solution

- Back the value with `TestWindowState`, still defaulting to `2.0`, so
existing tests are unchanged.
- Add `TestWindow::simulate_scale_factor_change`, which stores the new
factor and reports it through the resize callback — the same channel a
real DPI change reaches the window on — plus the matching wrappers on
`TestAppContext`, `VisualTestContext`, and `TestAppWindow`, mirroring
the `simulate_resize` family.
- Because the platform window owns the value, it survives later bounds
changes (unlike `set_scale_factor`); the test pins that.

## Testing

- Added `test_simulate_scale_factor_change` in
`crates/gpui/src/window.rs`: asserts the unchanged 2.0 default, that a
simulated change propagates to `Window::scale_factor()` through the
resize callback, and that it persists across a subsequent
`simulate_resize`.
- Reverting `scale_factor()` to the hardcoded `2.0` while keeping the
test reproduces the failure (see Showcase), so the test pins the
behaviour rather than passing vacuously.
- `cargo test -p gpui --lib` — 299 passed, 0 failed:

```
running 299 tests
...
test window::tests::test_simulate_scale_factor_change ... ok
...
test result: ok. 299 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.23s
```
- Tested on macOS. The change is confined to test-support code
(`platform/test/`, test contexts), which is platform independent, but I
have not built the Linux or Windows platform crates.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [ ] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Showcase

Before — the new test against the hardcoded `2.0`:

```
thread 'window::tests::test_simulate_scale_factor_change' panicked at crates/gpui/src/window.rs:7230:9:
assertion `left == right` failed
  left: 2.0
 right: 1.0

test result: FAILED. 0 passed; 1 failed
```

After — see the full run above.

Release Notes:

- N/A

---------

Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>
````

## Parents

- [1d25e83fe8f448cf5007a6877a63c6d146071ec4](https://git.cafe/gitcafe/zed/commit/1d25e83fe8f448cf5007a6877a63c6d146071ec4?format=markdown)

[Source at this commit](https://git.cafe/gitcafe/zed/tree/b95b188b59454bc2b01f08a54ab0182c4a55f141?format=markdown)