G9: SelectionGlow, a soft halo round the body's selection

SelectionGlow on|off (off; a pixel shell's, Chrome.selection_glow): each
block of selected rows in a pane body casts Lift's soft kind in the
selection's own colour, the block its caster (never touched) and a cursor
beside it joined to the caster; clipped to the bodies (never a tag or a
grip); at most 22% (§8.2), less where the text round it would drop below
min(its contrast, 4.5); fading in over 100 ms with an ease-out, on the
core's continuous path (shell_continuous); none while panes move. Blocks
come from the cells in the theme's selection colour (documented).

Tests: the halo's colour, its clip to the body, nothing at its first frame
and up to its cap after 100 ms, fading done. Gates as G8. Feel review:
.scratch/render/selection/.

1 files changed, 2 insertions, 0 deletions

diff --git a/docs/config.md b/docs/config.md
index db15289b..29671e4e 100644
--- a/docs/config.md
+++ b/docs/config.md
@@ -565,6 +565,8 @@ Lift auto        a shadow on a light page; on a dark one the others recede
 Lift off
 InactiveDim 30   the unfocused panes' text fades 30% toward its ground
 Motion smooth    off, crisp, smooth (the default), bouncy or playful
+SelectionGlow on a soft halo of the selection's colour round it in the body,
+                 fading in over 100 ms, never over a tag, a grip or the cursor
 CursorBlink on   the cursor blinks, solid while typing and half a second
                  after, eased at each edge, solid after 10 idle seconds
 GripWidth 150    the grip's button and the scrollbar under it, percent of the