Increase visual distinction for disabled state

Depending on the palette, PhantomStyle might not always provide enough
visual hints for disabled states. This commit tries to add some
additional visual distinction. This commit changes two things: it
decreases the outline contrast on disabled widgets, and removes the
specular shading on disabled widgets.

For the outline contrast, the Swatchy code which derives the outline
color from the window color is changed to use a smaller contrast value
when the QPalette's current color group is Disabled.

Many clickable widgets, like push buttons, have an inner ring of
highlighting which is called specular in the PhantomStyle code. This
commit changes the colors used for these specular highlights to be the
same as the colors that are underneath the highlights when the
QPalette's current color group is Disabled, thus making the specular not
visible. This means the painting operations for the specular roundrects
and rectangles are still occurring. It would be more efficient to have
code paths which don't draw the specular at all when a widget is
disabled, but this would require adding a number of additional code
paths in PhantomStyle in the widget drawing code, and I don't think
that's worth it at the moment.

It's probably worth revisiting if the Inactive state is ever changed to
also not show the specular, because redraws on multiple widgets might
occur when focusing and defocusing a window, and saving a few roundrect
draws would be worth it, even though it would add more code.

src/phantom/phantomstyle.cpp

@@ -352,6 +352,7 @@ using PhSwatchCache = QVarLengthArray<PhCacheEntry, Num_ColorCacheEntries>;
 Q_NEVER_INLINE void PhSwatch::loadFromQPalette(const QPalette& pal) {
   using namespace SwatchColors;
   namespace Dc = DeriveColors;
+  const bool isEnabled = pal.currentColorGroup() != QPalette::Disabled;
   QColor colors[Num_SwatchColors];
   colors[S_none] = QColor();
 
@@ -366,26 +367,37 @@ Q_NEVER_INLINE void PhSwatch::loadFromQPalette(const QPalette& pal) {
   colors[S_highlightedText] = pal.color(QPalette::HighlightedText);
   colors[S_scrollbarGutter] = Dc::gutterColorOf(pal);
 
-  colors[S_window_outline] = Dc::adjustLightness(colors[S_window], -0.1);
-  colors[S_window_specular] = Dc::specularOf(colors[S_window]);
+  // There's a chance that some widgets won't redraw when changing to and from
+  // the disabled state, causing the outline color in the frame buffer to be
+  // out of sync with what it should be. If we notice this problem, we can get
+  // rid of conditional color branching and try to do it some other way.
+  colors[S_window_outline] =
+      Dc::adjustLightness(colors[S_window], isEnabled ? -0.1 : -0.07);
+  colors[S_window_specular] =
+      isEnabled ? Dc::specularOf(colors[S_window]) : colors[S_window];
   colors[S_window_divider] = Dc::dividerColor(colors[S_window]);
   colors[S_window_lighter] = Dc::lightShadeOf(colors[S_window]);
   colors[S_window_darker] = Dc::darkShadeOf(colors[S_window]);
-  colors[S_button_specular] = Dc::specularOf(colors[S_button]);
+  colors[S_button_specular] =
+      isEnabled ? Dc::specularOf(colors[S_button]) : colors[S_button];
   colors[S_button_pressed] = Dc::pressedOf(colors[S_button]);
-  colors[S_button_pressed_specular] = Dc::specularOf(colors[S_button_pressed]);
+  colors[S_button_pressed_specular] =
+      isEnabled ? Dc::specularOf(colors[S_button_pressed])
+                : colors[S_button_pressed];
   colors[S_base_shadow] = Dc::overhangShadowOf(colors[S_base]);
   colors[S_base_divider] = Dc::dividerColor(colors[S_base]);
   colors[S_windowText_disabled] =
       pal.color(QPalette::Disabled, QPalette::WindowText);
   colors[S_highlight_outline] = Dc::adjustLightness(colors[S_highlight], -0.05);
-  colors[S_highlight_specular] = Dc::specularOf(colors[S_highlight]);
+  colors[S_highlight_specular] =
+      isEnabled ? Dc::specularOf(colors[S_highlight]) : colors[S_highlight];
   colors[S_progressBar_outline] = Dc::progressBarOutlineColorOf(pal);
   colors[S_inactiveTabYesFrame] =
       Dc::inactiveTabFillColorOf(colors[S_tabFrame]);
   colors[S_inactiveTabNoFrame] = Dc::inactiveTabFillColorOf(colors[S_window]);
   colors[S_inactiveTabYesFrame_specular] =
-      Dc::specularOf(colors[S_inactiveTabYesFrame]);
+      isEnabled ? Dc::specularOf(colors[S_inactiveTabYesFrame])
+                : colors[S_inactiveTabYesFrame];
   colors[S_inactiveTabNoFrame_specular] =
       Dc::specularOf(colors[S_inactiveTabNoFrame]);
   colors[S_indicator_current] = Dc::indicatorColorOf(pal, QPalette::Current);