With a large scrollback-limit (e.g. 1 GiB), resizing the window or adding
a split freezes the UI for a moment. Investigation showed the column
reflow rewrites the entire scrollback synchronously while holding the
terminal lock, which blocks the renderer and also the GTK main thread
(focus, mouse and key paths lock the terminal). Window drags trigger a
full reflow every 25 ms coalescing step, and the reflow inner loop scans
all tracked pins per cell. The design is inherited from upstream Ghostty.
TODO.md records the analysis and the options: a resize benchmark, reflow
fast paths, removing main-thread terminal locking on hot paths, deferring
history reflow until a drag settles, an incremental chunked reflow, and a
config escape hatch, with a recommended order. No behaviour change.
