[mitchellh]

This optimizes scrolling inside a scroll region (DECSTBM).

We're squeezing blood out of a rock here. There just isn't much improvement to be had given our architecture for this case, but there were a couple things found that result in modest gains.

Vtebench

You can see the modest gains here. Still behind Alacritty (very very close).

The changes

1. Stop creating scrollback for top-anchored regions on screens that don't retain scrollback. index() routed any full-width region with top == 0 through cursorScrollAbove(), which pushes the scrolled-out row into scrollback. Every scroll paid PageList.grow() plus amortized page pruning, which includes a 512 KB memset each time a page is recycled. These now use the in-place region scroll instead. CSI S gets the same routing fix. Result: 1.05x-1.49x on the bottom-anchored region workloads, 1.25x on alt-screen full-screen scrolling.

2. Add a specialized Screen.cursorScrollRegionUp() for the region scroll hot path. The previous fast path (PageList.eraseRowBounded) paid per-scroll bookkeeping that exceeded the actual row work. The new function is built around the invariant that the cursor sits on the bottom row of a full-width region. Result: 1.23x-1.24x on the top-anchored region workloads.

Benchmarks

workload	region (80 rows)	before	after	change
scrolling (control)	primary screen, no region	237 ms	235 ms	1.0x
scrolling_bottom_region	alt, rows 1-79	243 ms	231 ms	1.05x
scrolling_bottom_small_region	alt, rows 1-40	311 ms	208 ms	1.49x
scrolling_top_region	alt, rows 2-80	283 ms	229 ms	1.23x
scrolling_top_small_region	alt, rows 40-80	258 ms	208 ms	1.24x
alt screen full-screen scrolling	alt, no region	288 ms	230 ms	1.25x

LLM Notes

Assisted by Fable 5: it diagnosed the vtebench gap, wrote the benchmark harness payloads, profiled, and proposed hot paths. I manually wrote the hot path replacements and had it judge my work.