Add retained paints and shared text selection

docs/SCROLL.md

@@ -240,16 +240,16 @@ without a scroll-widget special case.
 The transcript registers the wheel **by hand rather than calling
 `LazySpan::scrollable()`**, and this is not an oversight. That helper also
 registers a finger drag driving the span's own `DragGesture`, and the
-transcript already has an arbiter — `Selection`, which must decide between
+transcript already has an arbiter — `SelectionController`, which must decide between
 panning and selecting text and so cannot let a second `DragGesture` see
 the same frames. `DragGesture`'s doc states the rule: one gesture, one
 arbiter, each frame delivered exactly once. The wheel handler registered
 here is identical to the helper's; only the drag differs.
 
-`Selection` is given the span by `set_scroll_area` after it exists (rows
-need a `Selection`, and the span needs the rows), and hands it committed
-pans and releases through `Scrollable::scroll`/`fling`. There is no
-wrapper widget: `TranscriptScreen::list` is the layout (`extent`,
+`SelectionController` is attached directly to the span and holds its weak
+handle, then hands committed pans and releases through
+`Scrollable::scroll`/`fling`. There is no wrapper widget:
+`TranscriptScreen::list` is the layout (`extent`,
 `key_at`, `jump_to_end`) *and* the position (`amt`, `fling`,
 `is_scrolling`).