perf(rendering): reshape the keyboard overlay only when its text changes (2.8x faster)

[notkainoa]

The keyboard overlay is 2.8x cheaper per frame. It rebuilt and reshaped its text on every frame while keys were showing, even though the text only changes when a new key appears. Now it keeps the shaped text and reshapes only when it changes.

	Before	After	Speedup
p50	19.1 µs	6.8 µs	2.8x
mean	19.5 µs	8.1 µs	2.4x
p95 (a new key appears)	26.5 µs	18.2 µs	1.5x

Runs on every frame of GPUI editor playback and export when the keyboard overlay is on.

What changed

• The shaped buffer is kept, and rebuilt only when the visible text, font size, wrap width, font or weight changes.
• The text color moved from the shaping attributes to the text area's default_color, which glyphon already uses for glyphs without their own color. Fades no longer force a reshape, and the pixels are the same.

Output is unchanged

• 29,615 rendered frames hash identical to main with perf(rendering): reuse shaped text overlay layouts between frames (6.7-8.2x faster) #2524, perf(rendering): reshape the keyboard overlay only when its text changes (2.8x faster) #2525 and perf(rendering): reshape captions only when their text or style changes (up to 7.6x faster) #2526 applied together. Covers all 14 text animations, text styles, mid-playback edits, keyboard and caption styles, playback, backward scrubbing, random seeks, pauses and resizes. A planted cache-key bug makes thousands of frames differ, so the check catches mistakes.
• New GPU test: on every frame of a typed segment, fade in and fade out included, the reused layer renders byte-identical pixels to a fresh one.
• A second GPU test changes the font size, family, weight, output height and output width on a warmed layer. Removing any field from the cache key makes it fail.
• cargo test -p cap-rendering --release --lib: 305 passed on current main. The GPU tests pass with --include-ignored.
• cargo clippy -p cap-rendering --all-targets -- -D warnings and cargo fmt are clean.

Benchmark setup

KeyboardLayer::prepare at 1920×1080: 3 typed commands at 60 fps. One warmup pass, then 3 measured passes pooled (466 visible frames per pass, 1,398 total). Each number is the median of 3 runs. Apple M1, release build, opt-level 3. "Before" is the same benchmark on main (3d19c5c).

CARGO_PROFILE_RELEASE_OPT_LEVEL=3 cargo test -p cap-rendering --release --lib benchmark_keyboard_layer_prepare -- --ignored --nocapture

Confidence Score: 5/5

The PR appears safe to merge; no blocking issue was found.

Summary

KeyboardLayer now keeps shaped text until the text, font size, wrap width, family, or weight changes.

• The keyboard overlay reuses shaped text between key and font changes.

The checkout lacked the previous review commit and a shared merge base. Review therefore covered the full endpoint diff.

_{Reviews (2) · Last reviewed commit: "test(rendering): check the keyboard layo..." · Reviewed by Greptile}

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[richiemcilroy]

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