The first-result viewport-repaint gate assumed only streamed
__partialJson placeholder shapes (SSH) could re-anchor; the write
renderer's collapsed pending preview paints a tail window from decoded
content, so its first partial result re-anchored to the top of the file
and left the committed tail rows stale above the new frame.
Resolve forceFirstResultViewportRepaint per renderer as a boolean or an
(args, options) predicate evaluated at paint time: write opts in when a
collapsed preview outgrew the streaming tail window, SSH stays scoped to
the streamed-placeholder shape it always covered.
Adopted from PR #4478 (roboomp) with an allocation-free line-count scan
and terminal-buffer regression coverage.
Fixes#4477
can1357