G5: lapis, 0x4200.cafe's manuscript, as a theme with its ornament

A native theme `lapis` (src/themes/lapis.zig) from web/theme/lapis.css:
a lapis-deep page, lapis tags with gold names, vellum text, vermilion
rubrics, gold rules and a vermilion tag rule, gold selection in lapis ink.
Its ornament is the theme's new `decor` (colors.Decor), drawn by a pixel
shell inside the chrome it already has (decision 1), all of it off every
focus indicator:

- page_dots: the site's gold dot grid (#e8c46a22, 22 px), drawn by the
  cell shader on page-coloured cells alone (flag 0x10000000), so it lies
  under the text and never under a selection, a tag or the cursor; fixed
  to the window, a page the panes lie on.
- rail_checker: the dither frame's gold and lapis checker in the scroll
  track (a decor kind, flag 0x08000000), anchored to its rail; the thumb
  stays solid gold.
- tag plaques (tag_border, tag_shadow, tag_stripe): every tag a raised
  plaque inside its own band, the site's trail: a 2px gold frame and a
  hard 5px vermilion shadow down and right, both within the band's rows
  and columns (down, what the slack under the text leaves), so no shadow
  reaches a body, a grip or the next pane. The focused pane's plaque is
  the code window's title bar, gold and lapis stripes of 2px, its words
  on plates of the focused ground; unfocused ones are plain; column and
  workspace tags take the object-label language (1px frame, 2px shadow
  toward the page). Drawn as decor under the cells (a group's under
  range), with blank tag cells see-through and the cells blended
  premultiplied (pipeline_over) on such frames; opaque windows only.
- title_shadow: the titles' vermilion offset shadow under a file name's
  letters, sampled in the cell shader inside the letters' own cells (flag
  0x08000000), none on a cursor.

Chrome carries the decor (decor_*, name_ink, active_name_ink); a theme
file folds a nested decor literal field by field. Left out, with why, in
docs/effects.md: the fonts, the ornaments, a decor crossfade, and in the
tty anything but the colours (a tag row has no spare cell for a shadow).
An empty column is the page. The effect id is masked to 27 bits (ui.vert/ui.frag) for the
fifth flag.

Tests: the cell flags land only on page ground and file-name cells, never
a cursor, nothing without decor; lapis's plaques stay inside their tag's
band, clear of every grip, only the focused one striped; its checker is in
the track; no other theme draws any; the separator and focus
tests cover lapis. Goldens: theme and themesel (the ring gains lapis after
acme). Gates as G4. Feel review: .scratch/render/lapis/ (lapis.png, lapis-2x.png,
tags-focused-vs-unfocused-2x.png, a PanelSlide at 60 fps and a quarter).

6 files changed, 49 insertions, 6 deletions

diff --git a/shaders/prebuilt/decor.frag.glsl b/shaders/prebuilt/decor.frag.glsl
index 3e2c286f..bb1ef0d6 100644
--- a/shaders/prebuilt/decor.frag.glsl
+++ b/shaders/prebuilt/decor.frag.glsl
@@ -8,6 +8,8 @@
 // colour rides in the cell's ground (a_bg), its coverage in a_fg.r, so the
 // result is the overlay pipeline's exactly.
 //
+// Checker (flag 0x08000000): two colours in squares, or in stripes, see main.
+//
 // Soft (flag 0x10000000): a rectangle's shadow, rim or glow blurred by a Gaussian,
 // in closed form (the product of two erf edges), on one quad grown by 3
 // sigma: no blur pass. Everything is in the quad's own 0..1 space, so a
@@ -51,6 +53,17 @@ void main() {
     if (v_clip.z >= v_clip.x && (gl_FragCoord.x < v_clip.x || gl_FragCoord.x >= v_clip.z ||
         gl_FragCoord.y < v_clip.y || gl_FragCoord.y >= v_clip.w)) discard;
     float opacity = clamp(u_ui.background_opacity, 0.0, 1.0);
+    // Checker (flag 0x08000000): squares of v_uv.x pixels from the quad's
+    // own corner, so it moves with its rail, in the ground's colour and the
+    // ink's (lapis's scroll track).
+    vec2 in_quad = v_cell_uv / max(vec2(dFdx(v_cell_uv.x), dFdy(v_cell_uv.y)), vec2(1e-6));
+    if ((v_effect.x & 0x08000000u) != 0u) {
+        ivec2 q = ivec2(floor(in_quad / max(v_uv.x, 1.0)));
+        // v_uv.y set: stripes across the quad (lapis's title bar), not squares.
+        int k = v_uv.y > 0.5 ? q.y : q.x + q.y;
+        o_col = vec4(((k & 1) == 0 ? v_bg : v_fg) * opacity, 1.0);
+        return;
+    }
     if ((v_effect.x & 0x10000000u) == 0u) {
         o_col = vec4(v_bg * opacity, v_fg.r);
         return;
diff --git a/shaders/prebuilt/decor.frag.spv b/shaders/prebuilt/decor.frag.spv
index 3256f2b1..bd565d03 100644
--- a/shaders/prebuilt/decor.frag.spv
+++ b/shaders/prebuilt/decor.frag.spv
diff --git a/shaders/prebuilt/ui.frag.glsl b/shaders/prebuilt/ui.frag.glsl
index dca224ba..62dbbfa2 100644
--- a/shaders/prebuilt/ui.frag.glsl
+++ b/shaders/prebuilt/ui.frag.glsl
@@ -10,6 +10,12 @@
 layout(set = 2, binding = 0) uniform sampler2D u_atlas;
 layout(std140, set = 3, binding = 0) uniform UiUniforms {
     float background_opacity;
+    // A theme's ornament (colors.Decor): the page's dots (rgb, strength),
+    // their geometry (period, solid radius, faded radius, in pixels), and a
+    // file name's shadow (rgb, offset in pixels).
+    vec4 decor_dots;
+    vec4 decor_dot_geom;
+    vec4 decor_shadow;
 } u_ui;
 
 layout(location = 0) in vec2 v_uv;
@@ -43,7 +49,7 @@ void main() {
     float bgA = (v_effect.x & 0x20000000u) != 0u ? 1.0
         : (v_effect.x & 0x40000000u) != 0u ? 0.0
         : clamp(u_ui.background_opacity, 0.0, 1.0);
-    uint effect = v_effect.x & 0x0fffffffu;
+    uint effect = v_effect.x & 0x07ffffffu;
     float progress = clamp(uintBitsToFloat(v_effect.y), 0.0, 1.0);
     uint cellCol = v_effect.w & 0xffffu;
     uint cellRow = v_effect.w >> 16;
@@ -52,7 +58,29 @@ void main() {
     // Ink over ground, both premultiplied: v_fg*a + (v_bg*bgA)*(1-a), which is
     // mix() with the ground already premultiplied. At bgA = 1 that is exactly
     // the mix(v_bg, v_fg, a) at alpha 1 this shader always wrote.
-    vec4 cell = vec4(mix(v_bg * bgA, v_fg, a), mix(bgA, 1.0, a));
+    vec3 ground = v_bg;
+    // 0x10000000: the page's ground under a pane body, dotted (lapis): a
+    // dot every period pixels, fixed to the window, under the text.
+    if ((v_effect.x & 0x10000000u) != 0u) {
+        float period = u_ui.decor_dot_geom.x;
+        vec2 d = mod(gl_FragCoord.xy, period) - 0.5 * period;
+        float dot_a = 1.0 - smoothstep(u_ui.decor_dot_geom.y, u_ui.decor_dot_geom.z, length(d));
+        ground = mix(ground, u_ui.decor_dots.rgb, u_ui.decor_dots.a * dot_a);
+    }
+    vec4 under = vec4(ground * bgA, bgA);
+    // Derivatives outside any branch: a pixel's step in the atlas and in
+    // the cell, for the shadow's offset.
+    vec2 step_uv = vec2(dFdx(v_uv.x), dFdy(v_uv.y)) * u_ui.decor_shadow.a;
+    vec2 step_cell = vec2(dFdx(v_cell_uv.x), dFdy(v_cell_uv.y)) * u_ui.decor_shadow.a;
+    // 0x08000000: a file name's letters cast a shadow down and right onto
+    // their own cell's ground (lapis), under the ink; what would fall past
+    // the cell's edge is not drawn.
+    if ((v_effect.x & 0x08000000u) != 0u) {
+        vec2 from = v_cell_uv - step_cell;
+        float s = all(greaterThanEqual(from, vec2(0.0))) ? texture(u_atlas, v_uv - step_uv).r : 0.0;
+        under = mix(under, vec4(u_ui.decor_shadow.rgb, 1.0), s);
+    }
+    vec4 cell = vec4(mix(under.rgb, v_fg, a), mix(under.a, 1.0, a));
 
     // 3 = data dissolve. Changed cells retain the frozen old cell until their
     // stable threshold, then atomically become the new cell. Exact endpoints
diff --git a/shaders/prebuilt/ui.frag.spv b/shaders/prebuilt/ui.frag.spv
index c8ee1604..d83d9c55 100644
--- a/shaders/prebuilt/ui.frag.spv
+++ b/shaders/prebuilt/ui.frag.spv
diff --git a/shaders/prebuilt/ui.vert.glsl b/shaders/prebuilt/ui.vert.glsl
index 5551f3f1..de22adee 100644
--- a/shaders/prebuilt/ui.vert.glsl
+++ b/shaders/prebuilt/ui.vert.glsl
@@ -14,9 +14,11 @@ layout(location = 3) in vec3 a_bg;
 layout(location = 4) in vec4 a_panel_rect;
 layout(location = 5) in vec4 a_present_rect;
 // effect id, eased-progress float bits, pane serial, local row/column packed
-// into one uint. The FOUR high effect bits are flags — the frozen-old cell
-// layer, a see-through cell background, an opaque block-cursor fill and a
-// soft shadow (decor.frag.glsl) — so the id is what is left below them; integer transport preserves the exact
+// into one uint. The FIVE high effect bits are flags — the frozen-old cell
+// layer, a see-through cell background, an opaque block-cursor fill, a soft
+// shadow (decor.frag.glsl) or dotted page ground (ui.frag.glsl), and a
+// checker (decor) or a file name's shadow (ui) — so the id is what is left
+// below them; integer transport preserves the exact
 // cross-backend hash.
 layout(location = 6) in uvec4 a_effect;
 // A clip rectangle in target pixels (x0, y0, x1, y1): the fragment shader
@@ -61,7 +63,7 @@ void main() {
         v_cell_uv = vec2(0.0, 1.0);
     }
 
-    uint effect = a_effect.x & 0x0fffffffu;
+    uint effect = a_effect.x & 0x07ffffffu;
     if (effect == 1u || effect == 2u || effect == 5u) {
         // NDC y points up, so a top-left/bottom-right box has a NEGATIVE y
         // delta. Preserve that sign or every local y is mirrored/outside.
diff --git a/shaders/prebuilt/ui.vert.spv b/shaders/prebuilt/ui.vert.spv
index b1e87ec1..af5b0738 100644
--- a/shaders/prebuilt/ui.vert.spv
+++ b/shaders/prebuilt/ui.vert.spv