When re-rendering content the user was already scrolled into, we want the
saved scroll position applied exactly when the real content first paints — not
before. Setting the origin up front instead paints it onto whatever placeholder
is currently in the view (e.g. the shorter buffer CopyContent left there),
which flickers: either a blank frame past the placeholder's end, or a jump to
the top when the task resets the origin at startup.
Add ViewBufferManager.ScrollToOriginYForNextTask: the next cmd/pty task then
(a) does not reset the view to the top at startup even though the command key
changed, so the placeholder stays put, (b) sizes its initial read to the saved
position so enough content is loaded to fill the view there, and (c) scrolls to
it as part of the first refresh, in the same paint that shows the real content.
This is the cmd/pty analogue of RenderStringWithScrollTask.
No caller sets it yet, so this is behaviour-preserving on its own.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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