Rules and chrome scale with the display's density; dark pages get visible separators and focus tints

Density: dp() in gui.zig draws every size given in pixels in logical
pixels (SDL's window display scale, rounded, at least one), as acme scales
Border, ButtonBorder and Scrollgap: the rules (rule_px), the tag rule, the
grip's ring, its dirty gap and mode bars, the rail's gap to the text and
its minimum, the thumb's inset, the rim, a bar cursor's minimum width,
underline and strikethrough thickness (the atlas is dropped on a scale
change), the rule-grab margin, and the Crt's scanline and mask periods
(pardesScale in the post prefix). Cell- and tagline-measured sizes stay:
the rail's width, Lift's blur and offsets, every motion; iResolution,
fragCoord and iMouse stay in the display's pixels, as ghostty's.
PARDES_TEST_SCALE forces a density in a harness (a test window is 1x
otherwise, whatever display it lands on) and sizes the test font with it.

Colours: a separator that stands off its page by less than 1.5:1 is lifted
toward the text to 1.6:1 (helix, atelier, paper; a theme with no page,
dark, is judged against #121212), and empty_col follows it. forge's own
rule already stands at 3.83:1; what hid it was density: 2 device pixels
on a 3x screen, now 6 (dp). A focused tag's tint closer than 1.5:1 to the
plain tag on a dark page (1.25:1 on a light one) is pushed further the way
it leans, as far as its text keeps its own contrast or 4.5: forge
1.23 -> 1.51 (#2d3440 -> #374253), and likewise dusk, orchard, ink,
lagoon, clay, harvest, spectrum, solarium, forge_soft; daybreak, atelier,
paper to 1.25. dark and helix gain a tint of their own (dark: #484566
under #c8c8c8, 1.54:1; helix: ramp 237 under 250, 1.65:1).

Tag caret: past a full row's end, the G3 glide box sits on the tag's right
edge, where the layer's caret and the pixel shell's are.

Tests: every native theme's separator >= 1.5 and every dark one has a
tint >= 1.25 with readable text; forge keeps its own border; the tag caret
glide box at a full row; at 2x the spine, tag rule, ring, rail start and
gap, and bar cursor double. Legacy-ThemeFile test strips helix's new pair.

Goldens: snapshot goldens change in colour rows only, the tints (orchard
#283a2f -> #314738 at boot, forge, helix and dark in theme/themesel).
GUI goldens: the harness's single Del of the renamed wide pane is refused
since ysqnkyoy (a rename counted as unsaved; fixed on the 9P line in
wkzlsqvy), so scenes 09-17 were recorded with that pane open and the file
was stale since the merge (08 and 18 also moved with ykvnltvs's full-row
tag wrap). The goldens here were recorded with that pane closed (as
wkzlsqvy will close it); against the parent's binary under the same
conditions they differ only in the tints: 01-13 forge's (#2d3440 ->
#374253, and the glyph edges on it), 14 ink's; 15-18 are byte-identical.
Until wkzlsqvy is merged, a run here fails 09-17. Each commit reproduces its own
recorded goldens on this machine (checked at kvtmqkyn). A test window is
1x unless PARDES_TEST_SCALE says otherwise, whatever display it opens on.
Prebuilt shaders refreshed from their sources.
Captures: .scratch/render/dark-borders/{before,after,compare,scale2,scale3}.

1 files changed, 53 insertions, 7 deletions

diff --git a/shaders/prebuilt/decor.frag.glsl b/shaders/prebuilt/decor.frag.glsl
index 4c2bd6d8..3e2c286f 100644
--- a/shaders/prebuilt/decor.frag.glsl
+++ b/shaders/prebuilt/decor.frag.glsl
@@ -1,24 +1,70 @@
 #version 450
 
-// Decor: a flat, hard-edged fill (a rule, a rail, a grip mark, a bar cursor)
-// that ui.vert.glsl places like a cell, its track's transition included. Its
-// colour rides in the cell's ground (a_bg) and its coverage in a_fg.r. The
-// pipeline blends it as the overlay pipeline does (backgroundLayerBlend), so
-// a fading rule shows through, and the result is that pipeline's exactly.
-// Ink (a cursor) is drawn with the opacity uniform at 1.
+// Decor, drawn by ui.vert.glsl like a cell, its track's transition included,
+// and blended as the overlay pipeline blends (backgroundLayerBlend). Ink (a
+// cursor) is drawn with the opacity uniform at 1.
+//
+// Flat: a hard-edged fill (a rule, a rail, a grip mark, a bar cursor). Its
+// colour rides in the cell's ground (a_bg), its coverage in a_fg.r, so the
+// result is the overlay pipeline's exactly.
+//
+// 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
+// track's transform carries it: sigma per axis in the uv, the caster's
+// offset from its shadow in fg.gb, the strength in fg.r. The caster's own
+// rectangle is left alone (it is drawn already), and the alpha is pre-warped
+// so black darkens in linear light, then dithered so it never bands.
 
 layout(std140, set = 3, binding = 0) uniform UiUniforms {
     float background_opacity;
 } u_ui;
 
+layout(location = 0) in vec2 v_uv;
 layout(location = 1) in vec3 v_fg;
 layout(location = 2) in vec3 v_bg;
+layout(location = 3) in vec2 v_cell_uv;
+layout(location = 4) flat in uvec4 v_effect;
 layout(location = 5) flat in vec4 v_clip;
 
 layout(location = 0) out vec4 o_col;
 
+// erf to about 4e-4 (Winitzki's approximation): smooth, cheap, odd.
+float erfApprox(float x) {
+    float x2 = x * x;
+    const float a = 0.147;
+    float e = sqrt(1.0 - exp(-x2 * (1.2732395 + a * x2) / (1.0 + a * x2)));
+    return x < 0.0 ? -e : e;
+}
+
+// How much of [lo, hi] a Gaussian of sigma s centred at x covers.
+float edge(float x, float lo, float hi, float s) {
+    float k = 0.70710678 / s;
+    return 0.5 * (erfApprox((x - lo) * k) - erfApprox((x - hi) * k));
+}
+
+float ign(vec2 p) {
+    return fract(52.9829189 * fract(dot(p, vec2(0.06711056, 0.00583715))));
+}
+
 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;
-    o_col = vec4(v_bg * clamp(u_ui.background_opacity, 0.0, 1.0), v_fg.r);
+    float opacity = clamp(u_ui.background_opacity, 0.0, 1.0);
+    if ((v_effect.x & 0x10000000u) == 0u) {
+        o_col = vec4(v_bg * opacity, v_fg.r);
+        return;
+    }
+    vec2 s = v_uv;
+    vec2 m = 3.0 * s;
+    vec2 p = v_cell_uv;
+    vec2 caster0 = m - v_fg.gb;
+    vec2 caster1 = 1.0 - m - v_fg.gb;
+    if (all(greaterThanEqual(p, caster0)) && all(lessThan(p, caster1))) discard;
+    float a = v_fg.r * edge(p.x, m.x, 1.0 - m.x, s.x) * edge(p.y, m.y, 1.0 - m.y, s.y);
+    // Black darkens in linear light; a light colour (a rim, a glow) is laid
+    // on as it is, within §8.2's 25%.
+    if (all(equal(v_bg, vec3(0.0)))) a = 1.0 - pow(max(1.0 - a, 0.0), 1.0 / 2.2);
+    a = clamp(a + (ign(gl_FragCoord.xy) - 0.5) / 255.0, 0.0, 1.0);
+    o_col = vec4(v_bg * opacity, a);
 }