PerKey canvas: symmetric hash stripes for unset cells

The "no change" hash overlay used a single black-or-white stripe
color picked by the base luminance, so the perceived average of an
unset cell was uniformly biased toward black or white instead of
sitting on the actual base color. That made dark base cells look
darker than they really are on the keyboard, and light ones lighter.

Draw two interleaved stripe sets at base ± offset (per channel),
spaced by half-period so the dark and light stripes alternate
evenly across the cell. Equal coverage of the two stripe colors
keeps the perceived average at base.

When a channel is too close to 0 or 1 to fit the full offset
(±0.22), halve the offset on the constrained side. The cell's
average then drifts at the limits but stays centered on base
everywhere else — verified visually across mid-tones, primaries,
and near-black/white bases.

The "no zone base color known" path keeps the previous neutral-grey
look unchanged; the average-preservation property only applies when
there is a base color to preserve.

lib/solaar/ui/perkey/canvas.py

@@ -229,9 +229,12 @@ class KeyboardCanvas(Gtk.DrawingArea):
             logger.debug("text rendering failed for %r: %s", label, e)
 
     def _fill_checker(self, cr, x, y, w, h) -> None:
-        # Diagonal hash for "no change" cells. Background uses the zone base
-        # color (what these cells actually display on the keyboard); stripes
-        # pick a black or white contrast based on luminance.
+        # Diagonal hash for "no change" cells. The background is the zone
+        # base color (what these cells actually display on the keyboard).
+        # Stripes alternate darker / lighter than base in equal measure so
+        # the cell's perceived average stays at the base color, instead of
+        # being uniformly biased toward black or white as a single-overlay
+        # stripe would.
         cr.save()
         self._round_rect(cr, x, y, w, h, 4)
         cr.clip()
@@ -240,26 +243,61 @@ class KeyboardCanvas(Gtk.DrawingArea):
             r = ((base >> 16) & 0xFF) / 255.0
             g = ((base >> 8) & 0xFF) / 255.0
             b = (base & 0xFF) / 255.0
+            cr.set_source_rgba(r, g, b, 1.0)
+            cr.rectangle(x, y, w, h)
+            cr.fill()
+            # Per-channel ±offset. When a channel is too close to 0 or 1 to
+            # fit the full offset, halve the offset on the constrained side
+            # (per spec: average drifts at the limits, but stays centered on
+            # base elsewhere).
+            offset = 0.22
+
+            def _shift(v: float) -> tuple[float, float]:
+                down_off = offset if (v - offset) >= 0.0 else offset / 2.0
+                up_off = offset if (v + offset) <= 1.0 else offset / 2.0
+                return max(0.0, v - down_off), min(1.0, v + up_off)
+
+            rd, ru = _shift(r)
+            gd, gu = _shift(g)
+            bd, bu = _shift(b)
+            # Interleave darker and lighter stripes (period = step per set,
+            # other-color set offset by step/2). Equal coverage of the two
+            # colors keeps the perceived average at base.
+            cr.set_line_width(1.5)
+            step = 6
+            half = step // 2
+            d_max = int(w + h)
+            cr.set_source_rgba(rd, gd, bd, 1.0)
+            d = -int(h)
+            while d <= d_max:
+                cr.move_to(x + d, y + h)
+                cr.line_to(x + d + h, y)
+                cr.stroke()
+                d += step
+            cr.set_source_rgba(ru, gu, bu, 1.0)
+            d = -int(h) + half
+            while d <= d_max:
+                cr.move_to(x + d, y + h)
+                cr.line_to(x + d + h, y)
+                cr.stroke()
+                d += step
         else:
-            r = g = 0.30
-            b = 0.32
-        cr.set_source_rgba(r, g, b, 1.0)
-        cr.rectangle(x, y, w, h)
-        cr.fill()
-        if base is not None and base >= 0:
-            lum = 0.299 * r + 0.587 * g + 0.114 * b
-            cr.set_source_rgba(0, 0, 0, 0.45) if lum > 0.55 else cr.set_source_rgba(1, 1, 1, 0.35)
-        else:
+            # No zone base color known — fall back to a neutral dark bg with
+            # medium-gray stripes; "average = base" doesn't apply since there
+            # is no expected color to preserve.
+            cr.set_source_rgba(0.30, 0.30, 0.32, 1.0)
+            cr.rectangle(x, y, w, h)
+            cr.fill()
             cr.set_source_rgba(0.55, 0.55, 0.60, 1.0)
-        cr.set_line_width(1.5)
-        step = 5
-        d_max = int(w + h)
-        d = -int(h)
-        while d <= d_max:
-            cr.move_to(x + d, y + h)
-            cr.line_to(x + d + h, y)
-            cr.stroke()
-            d += step
+            cr.set_line_width(1.5)
+            step = 5
+            d_max = int(w + h)
+            d = -int(h)
+            while d <= d_max:
+                cr.move_to(x + d, y + h)
+                cr.line_to(x + d + h, y)
+                cr.stroke()
+                d += step
         cr.restore()
 
     def _round_rect(self, cr, x, y, w, h, r) -> None: