G13: Parallax, lapis's dots drift with the text at a quarter of its speed

Parallax on|off (off, a pixel shell's; Chrome.fx bit 3). A body region
carries its first line (Region.line, a file's scroll or a terminal's grid
offset; the wire's region record grows a u32). The GUI offsets each
body's page dots by a quarter of its line in rows, and of G6's glide
offset while one runs, so the pattern drifts with the text and settles
with it; the cell shader reads it from the instance's serial slot, whose
per-cell dissolve noise G7 left unused (the dead noise code goes).

Tests: a scrolled body tells its line; the shift handed to the shader is
a quarter of it within the dots' period. Measured in a capture: four lines
move the dots 13 px (140 px / 4 mod 22). Gates as before, goldens
byte-identical. Feel review: .scratch/render/parallax/.

1 files changed, 9 insertions, 3 deletions

diff --git a/src/config.zig b/src/config.zig
index 22e65c46..d11cbe76 100644
--- a/src/config.zig
+++ b/src/config.zig
@@ -125,6 +125,7 @@ pub const leader_path = paths: {
     if (builtins.capabilities.lift) table.set(.SelectionGlow, null);
     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(.Lift, null);
     table.set(.Motion, null);
     table.set(.InactiveDim, null);
@@ -678,6 +679,9 @@ pub const Runtime = struct {
     /// G12: pane bodies darken faintly toward their edges (ambient
     /// occlusion, at most 2%), a pixel shell's.
     occlusion: bool = false,
+    /// 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,
     /// 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,
@@ -901,7 +905,7 @@ 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 };
+    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 };
     /// 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 };
@@ -963,6 +967,7 @@ pub const Runtime = struct {
         .{ .word = "SelectionGlow", .action = .{ .toggle = .selection_glow }, .availability = .lift, .doc = "A soft halo of the selection's colour round it in the body, on or off." },
         .{ .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 = "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." },
@@ -1039,9 +1044,9 @@ 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) {
+                } else if (field == .cursor_blink or field == .selection_glow or field == .hover_glow or field == .occlusion or field == .parallax) {
                     if (setting.availability != .lift)
-                        @compileError("CursorBlink, SelectionGlow, HoverGlow and Occlusion must use the lift capability (a pixel shell's)");
+                        @compileError("CursorBlink, SelectionGlow, HoverGlow, Occlusion and Parallax 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)
@@ -1699,6 +1704,7 @@ pub const Runtime = struct {
                     .selection_glow => "SelectionGlow on\n",
                     .hover_glow => "HoverGlow on\n",
                     .occlusion => "Occlusion on\n",
+                    .parallax => "Parallax on\n",
                 },
                 .shell => "Shell requested (new panes) fish\n",
                 .tagline_size => "TaglineSize 73\n",