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}.

10 files changed, 76 insertions, 18 deletions

diff --git a/shaders/post/crt.glsl b/shaders/post/crt.glsl
index 768157ef..970d61ee 100644
--- a/shaders/post/crt.glsl
+++ b/shaders/post/crt.glsl
@@ -69,9 +69,11 @@ void mainImage(out vec4 fragColor, in vec2 fragCoord) {
     // A beam is brightest on its line and falls off between lines; the
     // mask is three phosphor stripes a triad. Both average to one, so the
     // theme keeps its brightness.
-    float line = 0.5 + 0.5 * cos(fragCoord.y * TAU / SCAN_PERIOD_PX);
+    // Periods in logical pixels: the same look at any density.
+    float density = max(1.0, pardesScale);
+    float line = 0.5 + 0.5 * cos(fragCoord.y * TAU / (SCAN_PERIOD_PX * density));
     col *= 1.0 - scanStrength() * (line * line - 0.375) / 0.625;
-    col *= 1.0 + maskStrength() * cos(fragCoord.x * TAU / MASK_PERIOD_PX + vec3(0.0, TAU / 3.0, 2.0 * TAU / 3.0));
+    col *= 1.0 + maskStrength() * cos(fragCoord.x * TAU / (MASK_PERIOD_PX * density) + vec3(0.0, TAU / 3.0, 2.0 * TAU / 3.0));
 
     float vignette = 1.0 - vignetteStrength() * pow(clamp(r2 * 0.5, 0.0, 1.0), 1.6);
     float hum = 1.0 + humStrength() * sin((uv.y * 1.35 - iTime * HUM_HZ) * TAU);
diff --git a/shaders/post/prefix.glsl b/shaders/post/prefix.glsl
index e718a98f..7a50936e 100644
--- a/shaders/post/prefix.glsl
+++ b/shaders/post/prefix.glsl
@@ -41,10 +41,14 @@ layout(set = 3, binding = 0, std140) uniform Globals {
 
 // pardes's own: pardesOpaque is 1 when the window is opaque, so a pass
 // cannot punch holes in it through its alpha; pardesLevel is a bundled
-// pass's strength, 1 to 3 (`Crt 3`), and 0 for a user's file.
+// pass's strength, 1 to 3 (`Crt 3`), and 0 for a user's file; pardesScale
+// is the display's density (2 on a 2x screen), for a pass whose pattern is
+// in logical pixels. iResolution, fragCoord and iMouse stay in the display's
+// pixels, as ghostty's do.
 layout(set = 3, binding = 1, std140) uniform PardesPost {
     float pardesOpaque;
     float pardesLevel;
+    float pardesScale;
 };
 
 // A bundled pass's constant at levels 1, 2 and 3.
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);
 }
diff --git a/shaders/prebuilt/decor.frag.spv b/shaders/prebuilt/decor.frag.spv
index 7e162f5a..3256f2b1 100644
--- a/shaders/prebuilt/decor.frag.spv
+++ b/shaders/prebuilt/decor.frag.spv
diff --git a/shaders/prebuilt/post-crt.frag.glsl b/shaders/prebuilt/post-crt.frag.glsl
index c2cadfa2..7d2266c0 100644
--- a/shaders/prebuilt/post-crt.frag.glsl
+++ b/shaders/prebuilt/post-crt.frag.glsl
@@ -41,10 +41,14 @@ layout(set = 3, binding = 0, std140) uniform Globals {
 
 // pardes's own: pardesOpaque is 1 when the window is opaque, so a pass
 // cannot punch holes in it through its alpha; pardesLevel is a bundled
-// pass's strength, 1 to 3 (`Crt 3`), and 0 for a user's file.
+// pass's strength, 1 to 3 (`Crt 3`), and 0 for a user's file; pardesScale
+// is the display's density (2 on a 2x screen), for a pass whose pattern is
+// in logical pixels. iResolution, fragCoord and iMouse stay in the display's
+// pixels, as ghostty's do.
 layout(set = 3, binding = 1, std140) uniform PardesPost {
     float pardesOpaque;
     float pardesLevel;
+    float pardesScale;
 };
 
 // A bundled pass's constant at levels 1, 2 and 3.
@@ -143,9 +147,11 @@ void mainImage(out vec4 fragColor, in vec2 fragCoord) {
     // A beam is brightest on its line and falls off between lines; the
     // mask is three phosphor stripes a triad. Both average to one, so the
     // theme keeps its brightness.
-    float line = 0.5 + 0.5 * cos(fragCoord.y * TAU / SCAN_PERIOD_PX);
+    // Periods in logical pixels: the same look at any density.
+    float density = max(1.0, pardesScale);
+    float line = 0.5 + 0.5 * cos(fragCoord.y * TAU / (SCAN_PERIOD_PX * density));
     col *= 1.0 - scanStrength() * (line * line - 0.375) / 0.625;
-    col *= 1.0 + maskStrength() * cos(fragCoord.x * TAU / MASK_PERIOD_PX + vec3(0.0, TAU / 3.0, 2.0 * TAU / 3.0));
+    col *= 1.0 + maskStrength() * cos(fragCoord.x * TAU / (MASK_PERIOD_PX * density) + vec3(0.0, TAU / 3.0, 2.0 * TAU / 3.0));
 
     float vignette = 1.0 - vignetteStrength() * pow(clamp(r2 * 0.5, 0.0, 1.0), 1.6);
     float hum = 1.0 + humStrength() * sin((uv.y * 1.35 - iTime * HUM_HZ) * TAU);
diff --git a/shaders/prebuilt/post-crt.frag.spv b/shaders/prebuilt/post-crt.frag.spv
index 6599abd9..d6cc6c37 100644
--- a/shaders/prebuilt/post-crt.frag.spv
+++ b/shaders/prebuilt/post-crt.frag.spv
diff --git a/shaders/prebuilt/ui.frag.glsl b/shaders/prebuilt/ui.frag.glsl
index 0b099db3..dca224ba 100644
--- a/shaders/prebuilt/ui.frag.glsl
+++ b/shaders/prebuilt/ui.frag.glsl
@@ -43,7 +43,7 @@ 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 & 0x1fffffffu;
+    uint effect = v_effect.x & 0x0fffffffu;
     float progress = clamp(uintBitsToFloat(v_effect.y), 0.0, 1.0);
     uint cellCol = v_effect.w & 0xffffu;
     uint cellRow = v_effect.w >> 16;
diff --git a/shaders/prebuilt/ui.frag.spv b/shaders/prebuilt/ui.frag.spv
index 033c10ab..c8ee1604 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 c1779add..5551f3f1 100644
--- a/shaders/prebuilt/ui.vert.glsl
+++ b/shaders/prebuilt/ui.vert.glsl
@@ -14,9 +14,9 @@ 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 THREE high effect bits are flags — the frozen-old cell
-// layer, a see-through cell background and an opaque block-cursor fill — so
-// the id is what is left below them; integer transport preserves the exact
+// 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
 // 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 +61,7 @@ void main() {
         v_cell_uv = vec2(0.0, 1.0);
     }
 
-    uint effect = a_effect.x & 0x1fffffffu;
+    uint effect = a_effect.x & 0x0fffffffu;
     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 619a9690..b1e87ec1 100644
--- a/shaders/prebuilt/ui.vert.spv
+++ b/shaders/prebuilt/ui.vert.spv