fix(theme): #130 the chip moves, not the label; #129 pins the pair so it is checkable

Light-theme accent buttons sat at 2.72:1 — under AA and under even the 3:1
large-text floor. Bernardo: fix it. The plan on file ("darken the label")
turned out to be IMPOSSIBLE, and measuring all 24 palettes is what showed
it: on a saturated mid-tone accent nothing clears 4.5 — cream base 2.72,
dark text 1.65. That is why adw-gtk3 ships white-on-accent at ~2.7. The
chip has to move.

The chip is now the accent pulled AWAY from the label: toward white when
the label is dark, toward black when it is cream. One factor (0.70), only
the anchor flips, so "solid accent button + base label" stays the design
and just shifts tone. Worst dark 4.86 (nord/bad), worst light 5.59
(summer-day). The old 0.90 chip failed SEVEN themes (miasma/bad 3.43,
rose-pine 2.70) — never only a light-theme bug, contrary to the item.

#129 rides along, and the two are one idea. #98 shipped an invisible button
with every check green because the background was not a palette role —
adw-gtk3 computed it from currentColor. So the guard is not a CSS lint
(which would flag the harmless generic-button rule and get muted) but
arithmetic: pin both ends and the pair becomes checkable.
check-theme-contrast.py now models GTK's own mix(a,b,f) = a + (b-a)*f —
READ from gtkcsscolorvalue.c:234, not assumed — and asserts base-on-chip
>= 4.5 for all 24 themes. Its negative test reproduces the item's reported
2.72/2.74 exactly from an independent model, which is the best evidence
available that the arithmetic matches the pixels.

Invariant to keep: pin the pair, or pin neither. Both chips are pinned now
(suggested + .default + destructive), where #98 pinned only destructive.

tools/dialog-shot.nix deliberately not built: the checker covers the
numbers, and a VM that only ever confirms arithmetic is maintenance for
nothing. Render if a new widget class appears.

Also sweeps #127 with the latch finding (ff5017e): symptom 2 solved, not
DPMS — which raises the odds the mitigation treats the wrong cause.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/home/stylix.nix

@@ -16,6 +16,31 @@ let
   c = t.colors;
   hex = lib.removePrefix "#";
 
+  # Accent-button chips (#130). The label is the palette's base — dark on a
+  # dark theme, cream on a light one — so the chip has to be pulled AWAY from
+  # the label until the label is legible: toward white when the label is dark,
+  # toward black when it is cream. Same factor both ways; only the anchor
+  # flips, so "solid accent button, base label" stays the design and just
+  # shifts tone.
+  #
+  # Why a nudge at all: on a saturated mid-tone accent NOTHING clears 4.5 —
+  # neither the cream base (summer-day: 2.72) nor the dark text (1.65) — which
+  # is why upstream adw-gtk3 ships white-on-accent at ~2.7 and why "darken the
+  # label" was a dead end. The chip must move.
+  #
+  # 0.70 measured across all 24 palettes (tools/check-theme-contrast.py asserts
+  # it): worst dark 4.86 (nord/bad), worst light 5.59 (summer-day). Today's 0.90
+  # fails seven themes — miasma/bad 3.43, rose-pine 2.70 — so this is not only a
+  # light-theme fix, which is what the item assumed.
+  #
+  # GTK's mix(a, b, f) = a + (b - a) * f (gtkcsscolorvalue.c), i.e. f is the
+  # weight of the SECOND colour: 0.70 = 70% accent, 30% anchor. Read it that way
+  # or the ladder below looks inverted.
+  chipAnchor = if t.mode == "light" then "black" else "white";
+  # Each state pulls further from the label, so contrast only ever improves —
+  # rest is the worst case and the one the check pins.
+  chip = role: f: "mix(${chipAnchor}, ${role}, ${f})";
+
   # Map the Nomarchy palette onto base16 roles.
   base16 = {
     base00 = hex c.base;     # default background
@@ -116,31 +141,57 @@ in
             button:disabled, button:disabled label {
               color: alpha(#${base16.base05}, 0.5);
             }
-            /* Suggested/destructive sit on pastel accent/bad — dark label
-               is intentional and contrast-checked (7.0:1 suggested, 6.2:1
-               destructive on Boreal). */
+            /* Accent buttons: the palette's base as the label, on a chip
+               pulled away from it (#130 — see `chip` above for the why and
+               the measured numbers).
+
+               BOTH chips are pinned, not just destructive as in #98. Two
+               reasons, and the second is the durable one:
+                 1. adw-gtk3 builds the destructive background from
+                    currentColor — mix(@destructive_color,
+                    alpha(currentColor,…)) — so pinning the label alone drags
+                    the background with it and the button renders dark-on-dark
+                    (1.03:1, invisible). That is the #98 bug.
+                 2. A pinned label on an UNPINNED background is unknowable:
+                    no static check can see what it renders on, which is
+                    exactly why every check was green while #98 shipped. A
+                    pinned pair is arithmetic, and
+                    tools/check-theme-contrast.py now asserts it for all 24
+                    palettes. The rule for anything added here: pin the pair,
+                    or pin neither.
+
+               Enabled only — :disabled keeps the sheet's own chrome and
+               already renders legibly. .default is GTK's dialog default and
+               adw-gtk3 styles it as an accent button, which is why it shares
+               the suggested chip rather than inheriting a plain one. */
             button.suggested-action, button.suggested-action label,
             button.destructive-action, button.destructive-action label,
             button.default, button.default label {
               color: #${base16.base00};
             }
-            /* adw-gtk3 builds ONLY the destructive background from
-               currentColor — mix(@destructive_color, alpha(currentColor,…)) —
-               where suggested-action uses an opaque mix(white,@accent_bg…).
-               So pinning the label above drags the background down with it
-               and the button renders dark-on-dark (1.03:1, invisible). Pin
-               the background to the same opaque pastel ladder suggested-action
-               uses. Enabled only: :disabled keeps the sheet's dark chrome and
-               already renders legibly. */
+            button.suggested-action:not(.flat):not(:disabled),
+            button.default:not(.flat):not(:disabled) {
+              background-color: ${chip "@accent_bg_color" "0.70"};
+            }
+            button.suggested-action:not(.flat):not(:disabled):hover,
+            button.default:not(.flat):not(:disabled):hover {
+              background-color: ${chip "@accent_bg_color" "0.65"};
+            }
+            button.suggested-action:not(.flat):not(:disabled):active,
+            button.suggested-action:not(.flat):not(:disabled):checked,
+            button.default:not(.flat):not(:disabled):active,
+            button.default:not(.flat):not(:disabled):checked {
+              background-color: ${chip "@accent_bg_color" "0.55"};
+            }
             button.destructive-action:not(.flat):not(:disabled) {
-              background-color: mix(white, @destructive_bg_color, 0.9);
+              background-color: ${chip "@destructive_bg_color" "0.70"};
             }
             button.destructive-action:not(.flat):not(:disabled):hover {
-              background-color: mix(white, @destructive_bg_color, 0.85);
+              background-color: ${chip "@destructive_bg_color" "0.65"};
             }
             button.destructive-action:not(.flat):not(:disabled):active,
             button.destructive-action:not(.flat):not(:disabled):checked {
-              background-color: mix(white, @destructive_bg_color, 0.7);
+              background-color: ${chip "@destructive_bg_color" "0.55"};
             }
             button.suggested-action.flat, button.suggested-action.flat label {
               color: #${base16.base0D};