Problem:
Scrolling a 'scrollbind' window whose peer has a virt_lines block
taller than the window makes the peer jump back and forth.
Solution:
After scrollup()/scrolldown(), sync w_scbind_pos to the actual
get_vtopline() when the target was not clamped to buffer bounds.
Ref: esmuellert/codediff.nvim#519
AI-assisted
b781e95a24