_compensateScrollForMeasurementDelta re-found its viewport anchor only by
rawIdx and bailed with a bare 'if(!row) return' when that row was recycled
out of the render window. On long sessions containing multi-thousand-px
turns, a large scroll delta recycles the anchor row, so the estimated->measured
topPad swap (a scrollHeight change of tens of thousands of px) hit scrollTop
uncompensated and threw the viewport to the top -- a mobile scroll jump-back
that did not reproduce on desktop (where the anchor row stays rendered).
Fall back when the rawIdx row is gone: (1) re-find the same row by its stable
data-session-msg-idx and compensate by its measured offset delta; (2) if that
row is also unrendered, compensate by the top-spacer (topPad) height delta
captured before the re-render. _captureMessageViewportAnchor now records
topPadBefore so the fallback has a reference.
Adds tests/test_issue4346_vscroll_recycled_anchor_jumpback.py (Node-harness
behavioral tests). All fail on the pre-fix bare-return and pass on the fix;
existing vscroll suites stay green.
Relates to #4346.
ad1843f31b
