Implements the right-side chat scrollbar widget. Ports retail
UIElement_Scrollbar::UpdateLayout @0x4710d0 (thumb sizing + placement)
and HandleButtonClick @0x470e90 (step ±1 line, page on track click).
Dat element ids sourced from chat LayoutDesc 0x21000006 (base layout
0x2100003E): up-button sprite 0x06004C69, down-button 0x06004C6C, track
0x06004C5F, thumb middle 0x06004C63. Up/down buttons occupy the top and
bottom ButtonH (16px) regions of the widget height, matching element
positions Y=0 and Y=32 in the base scrollbar template.
Adds 6 pure ThumbRect tests (no GL): sizing, clamping to MinThumb,
position at start/mid/end, no-overflow full-fill.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
