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, 9 insertions, 3 deletions

diff --git a/src/config.zig b/src/config.zig
index 5734cd0c..d4efed0e 100644
--- a/src/config.zig
+++ b/src/config.zig
@@ -121,6 +121,7 @@ pub const leader_path = paths: {
     if (builtins.capabilities.window_blur) table.set(.WindowBlur, null);
     if (builtins.capabilities.ligatures) table.set(.Ligatures, null);
     if (builtins.capabilities.lift) table.set(.CursorBlink, null);
+    if (builtins.capabilities.lift) table.set(.SelectionGlow, null);
     if (builtins.capabilities.lift) table.set(.Lift, null);
     table.set(.Motion, null);
     table.set(.InactiveDim, null);
@@ -656,6 +657,9 @@ pub const Runtime = struct {
     /// solid while typing and for half a second after, eased at each edge,
     /// solid again after ten idle seconds.
     cursor_blink: bool = true,
+    /// G9: a soft halo round the selection in the body, fading in over
+    /// 100 ms, a pixel shell's (docs/effects.md).
+    selection_glow: bool = false,
     /// How long a message stays after the key or click that dismisses it.
     message_linger_ms: u16 = 800,
     /// How long its fall into the row takes, easing out (§8.1: arriving,
@@ -874,7 +878,7 @@ pub const Runtime = struct {
     /// The message timings a setting word sets, each in milliseconds.
     pub const MessageTime = enum { linger, fall, dissolve };
 
-    pub const Toggle = enum { colors, wrap, tree_context_tag_style, tag_bottom, debug, focus_tint, syntax_bold, verbose, message_animation, ligatures, cursor_blink };
+    pub const Toggle = enum { colors, wrap, tree_context_tag_style, tag_bottom, debug, focus_tint, syntax_bold, verbose, message_animation, ligatures, cursor_blink, selection_glow };
     /// A bundled post pass, a Shadertoy file in shaders/post/: the CRT, and
     /// G4's bloom, vignette and grain (docs/effects.md).
     pub const Scene = enum { crt, bloom, vignette, grain };
@@ -931,6 +935,7 @@ pub const Runtime = struct {
         .{ .word = "MessageAnimation", .action = .{ .toggle = .message_animation } },
         .{ .word = "Ligatures", .action = .{ .toggle = .ligatures }, .availability = .ligatures },
         .{ .word = "CursorBlink", .action = .{ .toggle = .cursor_blink }, .availability = .lift },
+        .{ .word = "SelectionGlow", .action = .{ .toggle = .selection_glow }, .availability = .lift },
         .{ .word = "Lift", .action = .lift, .availability = .lift },
         .{ .word = "Motion", .action = .motion },
         .{ .word = "InactiveDim", .action = .inactive_dim },
@@ -1006,9 +1011,9 @@ pub const Runtime = struct {
                 .toggle => |field| if (field == .ligatures) {
                     if (setting.availability != .ligatures)
                         @compileError("the ligatures toggle must use the ligatures capability");
-                } else if (field == .cursor_blink) {
+                } else if (field == .cursor_blink or field == .selection_glow) {
                     if (setting.availability != .lift)
-                        @compileError("CursorBlink must use the lift capability (a pixel shell's)");
+                        @compileError("CursorBlink and SelectionGlow must use the lift capability (a pixel shell's)");
                 } else if (setting.availability != null)
                     @compileError("unconditional toggles cannot carry a backend capability"),
                 else => if (setting.availability != null)
@@ -1661,6 +1666,7 @@ pub const Runtime = struct {
                     .message_animation => "MessageAnimation off\n",
                     .ligatures => "Ligatures off\n",
                     .cursor_blink => "CursorBlink off\n",
+                    .selection_glow => "SelectionGlow on\n",
                 },
                 .shell => "Shell requested (new panes) fish\n",
                 .tagline_size => "TaglineSize 73\n",