Selection was falling back to SGR-7 inverse (fg ↔ bg per cell), which
fragments over syntax-highlighted content — each amber/gold/dim/cornsilk
fg turned into a different bg stripe, producing the staircase look.
Now `useMainApp` calls `selection.setSelectionBgColor()` with a muted
navy (`#3a3a55`) on theme change. `setSelectionBg` in screen.ts replaces
just the bg cell-by-cell while preserving fg/bold/dim/italic, so the
highlight is one solid color across the whole drag range and the text
stays readable in its original color.
Skins can override via `selection_bg` in their color map.
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