acme looks like acme; every theme gets acme's button, scroll column and 2px rules

acme takes plan9port's own colours (acme.c iconinit, draw.h): selections
#eeee9e in the body and #9eeeee in the tag, black ink; exec and look sweeps
#aa0000 and #006600, white ink; a #99994c scrollbar under a #ffffea thumb;
black rules; a #8888cc tag/body rule and column button.

Every theme: the grip is acme's button, its tag row's full height, in one
scroll column with the scrollbar (rail_px, 12 at a 17px tagline, scaled,
times GripWidth, a percent, default 150; same x0/x1 to the pixel):
focused clean = solid box; focused dirty = box_dirty filled to a box ring
(acme's modbutton); unfocused clean = a box_border ring round the tag's
ground; unfocused dirty = box_dirty a pixel inside that ring. The grid, which
has no ring, marks dirty with a bold * on box_dirty in the grip's second cell
and fills an unfocused grip halfway to its ring. Regions carry a dirty flag.
Column buttons are the same shape, solid. 2px rules between columns and
stacked panes (inside the tag band's slack), none at the window edge; a 1px
tag/body rule halfway between tag and page.

Theme data: tag_sel_bg, sweep_bg/fg, tag_rule, rule_px, rail_px, box_border,
box_dirty. Chrome: rule_px, tag_rule, rail_px, grip_border, grip_focus_ring;
a comptime guard on Chrome's layout forces a wire bump once v8 ships.
Structural rules are exempt from the focus-indicator rule (docs/effects.md).

Tests: acme widths; stacked rule in the slack at two tagline sizes; every
grip-column pixel is band, button, ring or rule, and the button shares the
scrollbar's x0/x1 (4 metrics x top/bottom x focus x dirty); the four grip
states distinct in every native theme with a non-colour dirty cue in the
grid; wire round-trip of the new fields.

Shared files: src/colors.zig, src/surface.zig, src/body_layer.zig,
src/tagline.zig, src/detached/wire.zig, docs/themes.md, docs/config.md,
docs/effects.md, 21 snapshot goldens (the grip cells only),
test/gui-goldens.txt.

Motion flavours differ by design, and drive notice drops and pane moves

1 files changed, 18 insertions, 2 deletions

diff --git a/src/config.zig b/src/config.zig
index ed7bee92..24e058c7 100644
--- a/src/config.zig
+++ b/src/config.zig
@@ -118,6 +118,7 @@ pub const leader_path = paths: {
     if (builtins.capabilities.lift) table.set(.Lift, null);
     table.set(.Motion, null);
     table.set(.InactiveDim, null);
+    if (builtins.capabilities.lift) table.set(.GripWidth, null);
     if (builtins.capabilities.panel_transitions) {
         table.set(.PanelSlide, "as");
         table.set(.PanelZoom, "az");
@@ -629,6 +630,10 @@ pub const Runtime = struct {
     /// percent (0 off): the focused pane wins by comparison, its tag tint
     /// untouched. Kept above min(each cell's contrast, 4.5).
     inactive_dim: u8 = 0,
+    /// The grip's button and the scrollbar under it, one width, as a percent
+    /// of the theme's `rail_px` (acme's 12px Scrollwid at a 17px tagline):
+    /// a pixel shell's (docs/themes.md).
+    grip_width: u16 = 150,
     /// How the fx track's animations move (animation.Motion): off for
     /// reduced motion.
     motion: animation.Motion.Flavour = .smooth,
@@ -862,6 +867,7 @@ pub const Runtime = struct {
         motion,
         /// A percentage, 0 through 100.
         inactive_dim,
+        grip_width,
         transition: animation.Transition,
         scene: Scene,
         /// A Shadertoy file into the post chain or out of it; `off` empties
@@ -899,6 +905,7 @@ pub const Runtime = struct {
         .{ .word = "Lift", .action = .lift, .availability = .lift },
         .{ .word = "Motion", .action = .motion },
         .{ .word = "InactiveDim", .action = .inactive_dim },
+        .{ .word = "GripWidth", .action = .grip_width, .availability = .lift },
         .{ .word = "MessageLinger", .action = .{ .message_ms = .linger } },
         .{ .word = "MessageFall", .action = .{ .message_ms = .fall } },
         .{ .word = "MessageDissolve", .action = .{ .message_ms = .dissolve } },
@@ -960,8 +967,8 @@ pub const Runtime = struct {
                     @compileError("scene effects must use the scene-shader capability"),
                 // A toggle is plain state any shell can hold, except one for
                 // what only some shells draw: that one exists where they do.
-                .lift => if (setting.availability != .lift)
-                    @compileError("Lift must use the lift capability"),
+                .lift, .grip_width => if (setting.availability != .lift)
+                    @compileError("Lift and GripWidth must use the lift capability (a pixel shell's)"),
                 .toggle => |field| if (field == .ligatures) {
                     if (setting.availability != .ligatures)
                         @compileError("the ligatures toggle must use the ligatures capability");
@@ -1102,6 +1109,12 @@ pub const Runtime = struct {
                 if (percent > 100) return false;
                 state.inactive_dim = percent;
             },
+            .grip_width => {
+                const text = std.mem.trim(u8, argument orelse return false, " \t\r\n%");
+                const percent = std.fmt.parseInt(u16, text, 10) catch return false;
+                if (percent < 50 or percent > 300) return false;
+                state.grip_width = percent;
+            },
             .theme, .font => return false,
         }
         return true;
@@ -1277,6 +1290,7 @@ pub const Runtime = struct {
                 try out.print("{s} unsupported\n", .{setting.word}),
             .motion => try out.print("{s} {s}\n", .{ setting.word, @tagName(state.motion) }),
             .inactive_dim => try out.print("{s} {d}\n", .{ setting.word, state.inactive_dim }),
+            .grip_width => try out.print("{s} {d}\n", .{ setting.word, state.grip_width }),
         };
 
         if (context.startup_config_path) |path|
@@ -1547,6 +1561,7 @@ pub const Runtime = struct {
                 .lift => "rim",
                 .motion => "playful",
                 .inactive_dim => "35",
+                .grip_width => "200",
                 else => null,
             };
             try std.testing.expect(apply(&state, setting, argument));
@@ -1600,6 +1615,7 @@ pub const Runtime = struct {
                 .lift => "Lift rim\n",
                 .motion => "Motion playful\n",
                 .inactive_dim => "InactiveDim 35\n",
+                .grip_width => "GripWidth 200\n",
                 .theme, .font => unreachable,
             };
             try std.testing.expect(std.mem.indexOf(u8, report, expected) != null);