backend-write flushed output after every single draw-text/draw-rect
call, causing hundreds of individual flushes per frame. This caused
visible flicker on slow terminals.
Remove finish-output from backend-write — all critical flush points
(initialize-backend, shutdown-backend, enable-mouse, enable-bracketed-paste,
end-sync) already call finish-output explicitly.
DECICM sync (begin-sync/end-sync) wraps every frame boundary,
making the frame render atomically with a single flush at end-sync.
Amr Gharbeia
bb579be207