Terminal Profiling Plan

Context

• Terminal profiler (assets/lua/prof-terminal.fnl) shows ~33% in TerminalRenderer.paint and ~30% in font-state resolution/writes, driven by full-screen dirty regions when PTY floods output.
• Each cell walks all font states and rewrites background/glyph/underline buffers every frame; long outputs repeatedly mark the whole viewport dirty, stalling the main thread.

Goals

• Reduce per-cell work during heavy output so the UI stays responsive.
• Keep correctness for scrollback, alt-screen, and cursor rendering.

Plan

• Short term
  • Cache the resolved font state per cell and skip iterating each-font-state when only one applies; avoid per-cell table scans.
  • Skip buffer writes for unchanged cells within a region (track a hash or reuse vterm dirty rectangles to only touch modified cells).
  • Guard PTY drain-induced redraws with a max cells-per-frame or time budget; if exceeded, spread work across frames and keep input processing alive.
• Medium term
  • Rework get_dirty_regions consumption to batch by rows/columns and reuse line-for-viewport results instead of recomputing per cell.
  • Separate background and glyph passes so background-only changes (e.g., reverse) do not force glyph buffer rewrites.
  • Add a fast-path for “full redraw” that precomputes row slices and uses memcpy-like buffer fills instead of per-cell Fennel loops.
• Validation
  • Extend assets/lua/prof-terminal.fnl with a mode that simulates a log spammer (large scrollback) and measure frame time before/after.
  • Add a Lua test that drives TerminalWidget with synthetic dirty regions to assert we only touch changed cells and preserve scrollback/follow-tail behavior.
  • Manually run a long-output command (e.g., yes | head -n 50000) in-app to confirm no UI stall and cursor remains responsive.