# Objective

`gpui_macos` implements `PlatformWindow::render_to_image` — the
MetalRenderer samples an offscreen target, so a **hidden** window can be
captured. `gpui_windows` does not implement it, so the method falls
through to the trait default and returns `Err("render_to_image not
implemented for this platform")`.

That leaves Windows consumers (tests, headless screenshot harnesses) on
`PrintWindow`-style workarounds, which require the window to actually be
on screen: a DirectComposition swap chain that never presents has no
frame to read, so you get a visible flash — or an off-screen parking
trick — for every capture.

This closes the macOS/Windows gap. Linux (`gpui_linux`) still returns
the trait default; a Blade/wgpu readback is the obvious follow-up but is
not in this PR.

## Solution

The DirectX analogue of the Metal path:

- `DirectXRenderer::draw`'s clear, scene upload and batch encoding are
factored into a shared `render(scene, background_appearance)`. `draw` is
now `render` + `present`; `render_to_image` is `render` + readback.
Nothing about how a frame is produced is duplicated between them, so the
two cannot drift.
- `render_to_image` renders into the **existing** render target —
created at window construction, so no window need ever be shown — then
copies it into a `D3D11_USAGE_STAGING` texture, `Map`s it, and converts
BGRA → RGBA. `RowPitch` padding is honoured per row; the copy loop
carries a safety comment.
- It refuses to run while `skip_draws` is set. That flag marks a pending
device-lost recovery, where the atlas still holds tile references from
the previous device — drawing before the forced re-render rebuilds them
panics in `DirectXAtlasState::texture` (the case
`WindowState::force_render_pending` documents). Returning an error beats
panicking in a capture harness.
- Gated on `cfg(any(test, feature = "test-support"))` to match the trait
method. `gpui_platform`'s `test-support` now forwards to
`gpui_windows/test-support`, the way it already does for `gpui_macos`.

No behaviour change outside `test-support`/`cfg(test)` builds beyond the
`render` factoring, which is a pure code move.

## Testing

- **Windows, functional:** an earlier revision of this patch has been in
production use since June in the `--screenshot` harness of a GPUI app I
ship ([Ferail](https://github.com/jonx/Ferail)), capturing a hidden
window (`show: false`) with no flash. BGRA→RGBA output was verified
against the on-screen rendering.
- **This exact revision** has been cross-checked from macOS only: `cargo
clippy -p gpui_windows --no-default-features --features test-support
--target x86_64-pc-windows-msvc -- -D warnings` is clean, both with and
without `test-support` (clang-cl + `xwin`; `--no-default-features` skips
only the `windows-manifest` embed-resource step, which is irrelevant
here). I'm relying on CI for a native Windows build of this revision —
happy to report back from a Windows host if that's a blocker for review.
- **Not covered by an automated test.** The behaviour needs a live D3D11
device, so it can only be exercised by a Windows-hosted test. Happy to
add one under `gpui_windows` if you'd like it — say the word and I'll
push it rather than guess at the shape you want.
- **Reviewers:** the interesting part is the staging-texture readback in
`directx_renderer.rs`. Worth a second pair of eyes on the `skip_draws`
guard and on premultiplied alpha (the swap chain is
`DXGI_ALPHA_MODE_PREMULTIPLIED`, so non-opaque captures come back
premultiplied — same as the macOS path, but it does mean
semi-transparent regions are not straight-alpha).

## 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 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) — n/a, no UI surface
- [ ] Tests cover the new/changed behavior — see Testing; needs a
Windows host, happy to add
- [x] Performance impact has been considered and is acceptable — the
readback path is `test-support`-only; `draw` is unchanged apart from the
`render` factoring

## Showcase

<!-- TODO: drop in the headless Windows capture (and the macOS one
beside it) -->

---

Release Notes:

- N/A

Co-authored-by: Lukas Wirth <lukas@zed.dev>