G14: Afterglow, changed text glows a moment and fades

Afterglow on|off (off, a pixel shell's; Chrome.fx bit 4): the GUI keeps
the grid it last drew and when each cell changed; a body cell whose text
changed glows in the theme's accent, at most 18% and under the text's
contrast floor, fading with an ease-out over 250 ms, one flat rectangle a
run of cells that changed together; never a tag or a grip, the cursor or a
selection; a scrolled body (its first line moved) starts over rather than
lighting up whole. On the continuous path while anything glows.

Tests: three changed cells glow as a run of two (the cursor's left out),
gone by 250 ms, a scroll lights nothing. Gates as before; goldens
byte-identical. Feel review: .scratch/render/afterglow/.

1 files changed, 9 insertions, 3 deletions

diff --git a/src/config.zig b/src/config.zig
index d11cbe76..2a764118 100644
--- a/src/config.zig
+++ b/src/config.zig
@@ -126,6 +126,7 @@ pub const leader_path = paths: {
     if (builtins.capabilities.lift) table.set(.HoverGlow, null);
     if (builtins.capabilities.lift) table.set(.Occlusion, null);
     if (builtins.capabilities.lift) table.set(.Parallax, null);
+    if (builtins.capabilities.lift) table.set(.Afterglow, null);
     if (builtins.capabilities.lift) table.set(.Lift, null);
     table.set(.Motion, null);
     table.set(.InactiveDim, null);
@@ -682,6 +683,9 @@ pub const Runtime = struct {
     /// G13: a theme's page pattern under the bodies (lapis's dots) moves
     /// with the text at a quarter of its speed, a pixel shell's.
     parallax: bool = false,
+    /// G14: text that changes glows for a moment in the theme's accent and
+    /// fades, a pixel shell's.
+    afterglow: 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,
@@ -905,7 +909,7 @@ 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, selection_glow, hover_glow, occlusion, parallax };
+    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, hover_glow, occlusion, parallax, afterglow };
     /// 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 };
@@ -968,6 +972,7 @@ pub const Runtime = struct {
         .{ .word = "HoverGlow", .action = .{ .toggle = .hover_glow }, .availability = .lift, .doc = "A soft underline of light under the word a look-hover would open, on or off." },
         .{ .word = "Occlusion", .action = .{ .toggle = .occlusion }, .availability = .lift, .doc = "Pane bodies darken faintly toward their edges, on or off." },
         .{ .word = "Parallax", .action = .{ .toggle = .parallax }, .availability = .lift, .doc = "A theme's page pattern moves with the text at a quarter of its speed, on or off." },
+        .{ .word = "Afterglow", .action = .{ .toggle = .afterglow }, .availability = .lift, .doc = "Text that changes glows for a moment and fades, on or off." },
         .{ .word = "Lift", .action = .lift, .availability = .lift, .doc = "How the focused pane stands off the page: off, shadow, rim or auto." },
         .{ .word = "Motion", .action = .motion, .doc = "How animations move: off, crisp, smooth, bouncy or playful." },
         .{ .word = "InactiveDim", .action = .inactive_dim, .doc = "Fade the unfocused panes' text by a percentage, 0 through 100, 0 off." },
@@ -1044,9 +1049,9 @@ 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 or field == .selection_glow or field == .hover_glow or field == .occlusion or field == .parallax) {
+                } else if (field == .cursor_blink or field == .selection_glow or field == .hover_glow or field == .occlusion or field == .parallax or field == .afterglow) {
                     if (setting.availability != .lift)
-                        @compileError("CursorBlink, SelectionGlow, HoverGlow, Occlusion and Parallax must use the lift capability (a pixel shell's)");
+                        @compileError("CursorBlink, SelectionGlow, HoverGlow, Occlusion, Parallax and Afterglow 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)
@@ -1705,6 +1710,7 @@ pub const Runtime = struct {
                     .hover_glow => "HoverGlow on\n",
                     .occlusion => "Occlusion on\n",
                     .parallax => "Parallax on\n",
+                    .afterglow => "Afterglow on\n",
                 },
                 .shell => "Shell requested (new panes) fish\n",
                 .tagline_size => "TaglineSize 73\n",