When a task renders different content to a view (a new task key), the
view's scroll origin is reset to the top via onNewKey. That ran on the
task's own goroutine, racing the UI thread, which reads the origin
(OriginY) while laying out and drawing the view -- the single largest
source of view-render data races.
Give ViewBufferManager a bounce primitive (onUIThread) that runs a
function on the UI thread and waits for it, and reset the origin through
it. This is the first use of the primitive; subsequent commits route the
rest of the task's view mutations through it too, so that the view is
only ever touched on the UI thread. It runs as background work
(OnUIThreadAndWaitBackground) so rendering doesn't count towards the app
being busy, matching how the render's gocui task is already created.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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