- Add `DatFont` property (UiDatFont?): when set, OnDraw uses
ctx.DrawStringDat + datFont.MeasureWidth for all transcript lines;
BitmapFont path unchanged as fallback when DatFont is null.
- Cache `_lastDatFont` alongside `_lastFont` so HitChar hit-tests the
same advance source that drew the last frame.
- HitChar prefers `_lastDatFont` (via UiDatFont.GlyphAdvance) over
`_lastFont` (via bf.Advance) for column resolution, keeping
drag-select and Ctrl+C accurate with the dat font.
- Scroll event handler uses DatFont?.LineHeight first, so the wheel
quantum stays correct when the dat font has a different line height.
- WheelLines 3f → 1f: retail UIElement_Text::HandleMouseWheel
(@0x471450) advances one line per notch; our 3-line quantum was
wrong.
- Add UiChatViewDatFontTests: pins GlyphAdvance formula
(Before+Width+After = 10) and CharIndexAt dat-advance integration.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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