[OptimistOtaku]

Caption synchronization clones every source caption and word into a temporary sanitized transcript before projecting any of it. A cut recording pays for thousands of string allocations even when most words are discarded. GPUI runs this path when synchronizing captions after timeline edits and when opening the editor.

Project directly from the borrowed source captions, applying the same f32 word-duration cap when calculating word bounds and overlaps. Only retained output words need their text cloned. Projects without clip mappings still return the same clamped captions. No cache, format, timing, or public API changes.

Measurements

Existing caption-projection-benchmark example, upstream 1965b3179 versus this change. Windows x86_64, Ryzen 5 5600H, Rust 1.88.0 GNU, repository release profile (opt-level=s, thin LTO). Five alternating process runs per version; medians of 25 samples per case, each averaging 50 synchronizations after warmups. All generated-track fingerprints match.

Source / retained clips	Before	After	Speedup
One hour / one full clip	2.947 ms	1.975 ms	1.49×
Ten minutes / 20 four-second clips	0.122 ms	0.033 ms	3.65×
One hour / 60 four-second clips	0.869 ms	0.155 ms	5.61×

These measure complete in-memory synchronize_captions, including projection, override preservation, sorting, and output allocation. They exclude file loading, UI dispatch, and rendering; this is not an overall FPS or editor-startup claim.

Reproduce the API comparison by building both revisions:

cargo build -p cap-project --locked --release --example caption-projection-benchmark
target/release/examples/caption-projection-benchmark 50

The committed ignored benchmark also compares projection directly against the exact old function, including reversed-clip controls:

cargo test -p cap-project --locked --release --lib benchmark_caption_projection_allocations -- --ignored --nocapture

Validation

• cargo fmt --all
• cargo check -p cap-project --locked
• cargo test -p cap-project --locked --lib: 168 passed, three benchmarks ignored by default.
• cargo clippy -p cap-project --all-targets --locked -- -D warnings
• git diff --check

Differential tests compare complete projection results, including float bits, against the previous implementation. Cases include capped words, unsorted words, empty and reversed intervals, repeated/reordered/overlapping clips, take offsets, transitions, fullscreen holds, absent mappings, and NaN/infinite timestamps. Existing migration, style-preservation, and serialization tests pass. The native GPUI app was not launched locally.

Confidence Score: 5/5

The PR appears safe to merge.

Summary

Caption projection now borrows source captions instead of cloning a temporary clamped copy.

• Caption projection skips copying words that edited clips discard.

No actionable issues were found. Tests were not run during this review.

_{Reviews (1) · Last reviewed commit: "perf(project): avoid cloning captions be..." · Reviewed by Greptile}

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