Previously the editor and palette listened to Gtk.Settings
notify::gtk-theme-name (and notify::gtk-application-prefer-dark-theme)
to re-render their themed icons on theme switch. Two problems:
1. The initial icon load happened during widget construction, before
the buttons were attached to the toolbar — so the style context
resolved to a default (often white) foreground rather than the
actual theme text color. Icons showed up white until the first
theme-change event.
2. Settings notify fires *before* GTK's CSS engine re-resolves styles
for the new theme. Reading the style context's foreground from
that handler returned the previous theme's color, so toggling
light <-> dark left both states settling on the same shade.
Move both responsibilities into a new attach_themed_icon helper in
_icons.py: it does the initial load, connects to the *button's own*
style-updated signal, and rebuilds the icon on each emission. That
signal fires *after* CSS resolution (both on first realize and on
runtime theme switches), so the foreground we read is always the
current one.
A per-button color-key guard skips the rebuild when the resolved
foreground hasn't changed, so unrelated style-updated emissions
(hover, focus, active) don't trigger needless re-renders.
The handler is connected to the button itself, so GTK cleans it up
when the button is destroyed; both editor.py and palette.py drop
their bespoke Gtk.Settings handler bookkeeping.
The "Unset" toggle button next to the color picker used a custom
HashSwatch drawing area that mirrored the canvas's diagonal-hash
pattern for unset cells. The pattern was visually noisy at button
size and tied the button's appearance to the current zone base
color (the swatch had to be told the base color via
set_zone_base_color so it could redraw).
Use the Tabler "palette-off" symbolic icon instead. It reads as
"clear / no paint" at a glance, is independent of the zone base
color, and matches the icon style of the tool buttons on the other
end of the toolbar.
Extract the icon loader (themed_icon_image, ensure_icon_path) from
editor.py into a new private module _icons.py so palette.py can
reuse it; the new palette tracks its own Gtk.Settings notify signals
to re-render the icon on theme switches, and disconnects them via a
new Palette.shutdown() called from the editor's shutdown.
Adds MIT-licensed Tabler palette-off icon under share/solaar/icons/,
see THIRD_PARTY.md.
Ken Sanislo