Neil
c36dd5f4c7
perf(terminal): skip the partial-escape-tail walk on ESC-free PTY chunks ( #18748 )
...
`advancePartialEscapeTail` runs once per PTY chunk, on the main thread, for every terminal —
visible, hidden or parked — inside `HeadlessEmulator`'s write path. It unconditionally
concatenated the pending tail with the whole chunk and then walked the result one code unit at
a time through a VT500 state machine.
`extractPartialEscapeTail` only leaves `ground` on an ESC byte, so with no pending tail and no
ESC in the chunk the answer is always ''. Taking that case up front skips both the full-chunk
concat and the walk. `String.prototype.includes` is a native scan, so the gate costs
essentially nothing on the chunks it does not short-circuit.
This is the same gate its two neighbours on the very same ingest path already apply —
`TerminalOscCwdTitleScanner.scan` and `TerminalMouseModeMirror.scan`, both carrying a comment
citing their measured share of a 2.2x ingest regression. This call was simply missed.
Measured by `pnpm bench:terminal-partial-escape-tail` over 640 x 16 KB chunks (10 MB), median
of 7 rounds:
| stream shape | before | after | |
| --- | --- | --- | --- |
| ESC-free (build logs, `cat`, piped output) | 40.3 ms | 0.19 ms | 213x |
| SGR-coloured output (gate does not apply) | 30.6 ms | 32.1 ms | 1.0x |
The benchmark proves equivalence over a 226-case corpus before timing, and a new unit test
pairs every pending-tail state the scanner can be left in against every chunk shape, asserting
the gate is indistinguishable from the unconditional fold.
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