perf(gpui): reduce screenshot editor startup time

[dovidmoishe]

Screenshot capture now passes retained pixels directly to the editor, avoiding a second PNG decode and showing an initial preview while GPU rendering initializes.

Adds opt-in experiments for preparing the editor window during capture and warming GPU resources. Hidden preparation is limited to Windows/macOS.

With both experiments enabled and warmed up, capture-to-render median fell from 909ms to 483ms (47%) across four samples per mode.

Validation: 63 screenshot-related tests passed. All four Windows modes passed visibility, export, close, and reopen checks, with identical fresh/reopened exports. Native Linux/macOS testing remains outstanding.

Confidence Score: 5/5

The PR appears safe to merge; the previous large-screenshot saving issue is fixed, and no new blocking issues were found.

Summary

Speeds up screenshot editor startup by passing captured pixels directly to the editor.

• Fresh screenshots reach the editor without another PNG decode.
• Screenshot rendering can reuse GPU layers warmed ahead of time.
• The editor window can be prepared while a screenshot is being captured.

No new actionable issues were found. Checks were based on source review; native UI checks and tests were not run.

_{Reviews (2) · Last reviewed commit: "fix(gpui): preserve oversized screenshot..." · Reviewed by Greptile}

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[greptile-apps]

Large screenshots are not saved

capture_and_write now rejects images over 16,384 pixels in either direction before saving the PNG, even for save-only and clipboard-only captures. If a window spans enough high-resolution displays to exceed that size, the X11 capture path uses its full physical bounds and reaches this check. Previously the PNG was saved even if the editor could not open it; now the user gets neither a file nor a clipboard image.

Keep the size limit at the editor boundary so larger screenshots can still be saved and copied.

Prompt To Fix With AI

This is a comment left during a code review.
Path: apps/desktop-gpui/src/screenshot.rs
Line: 204-207

Comment:
**Large screenshots are not saved**

`capture_and_write` now rejects images over 16,384 pixels in either direction before saving the PNG, even for save-only and clipboard-only captures. If a window spans enough high-resolution displays to exceed that size, the X11 capture path uses its full physical bounds and reaches this check. Previously the PNG was saved even if the editor could not open it; now the user gets neither a file nor a clipboard image.

Keep the size limit at the editor boundary so larger screenshots can still be saved and copied.

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For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

[dovidmoishe]

@greptileai