Bug 1869525 - Watch KDE's colors.css and trigger theme recomputation when it mutates. r=stransky

Consider the following situation:

 * We're using the Breeze (light) theme.
 * We switch to dark mode.

When the KDE system theme changes, we get a color scheme change
notification. However by the time we get it, the gtk colorreload module
that KDE uses to communicate its colors to GTK[1] hasn't processed the
change yet.

So we end up thinking that Breeze is still light, (and even
special-casing that wouldn't work, because we still get the light
colors). So we fall back to Adwaita-dark, and by the time the file
actually changes we don't get any other notification. I don't think
there's another thing to watch here, unfortunately, but if there is we
should do it. I guess we could patch the color-reload module to send
some sort of signal to the screen / settings objects?

[1]: https://invent.kde.org/plasma/kde-gtk-config/-/blob/master/color-reload-module

Differential Revision: https://phabricator.services.mozilla.com/D196164

widget/gtk/GRefPtr.h

@@ -27,6 +27,7 @@ struct GObjectRefPtrTraits {
 
 GOBJECT_TRAITS(GtkWidget)
 GOBJECT_TRAITS(GFile)
+GOBJECT_TRAITS(GFileMonitor)
 GOBJECT_TRAITS(GMenu)
 GOBJECT_TRAITS(GMenuItem)
 GOBJECT_TRAITS(GSimpleAction)

widget/gtk/nsLookAndFeel.cpp

@@ -215,6 +215,17 @@ nsLookAndFeel::nsLookAndFeel() {
         },
         this, nullptr);
   }
+  if (IsKdeDesktopEnvironment()) {
+    GUniquePtr<gchar> path(
+        g_strconcat(g_get_user_config_dir(), "/gtk-3.0/colors.css", NULL));
+    mKdeColors = dont_AddRef(g_file_new_for_path(path.get()));
+    mKdeColorsMonitor = dont_AddRef(
+        g_file_monitor_file(mKdeColors.get(), G_FILE_MONITOR_NONE, NULL, NULL));
+    if (mKdeColorsMonitor) {
+      g_signal_connect(mKdeColorsMonitor.get(), "changed",
+                       G_CALLBACK(settings_changed_cb), NULL);
+    }
+  }
 }
 
 nsLookAndFeel::~nsLookAndFeel() {

widget/gtk/nsLookAndFeel.h

@@ -17,6 +17,8 @@ enum WidgetNodeType : int;
 struct _GtkStyle;
 typedef struct _GDBusProxy GDBusProxy;
 typedef struct _GtkCssProvider GtkCssProvider;
+typedef struct _GFile GFile;
+typedef struct _GFileMonitor GFileMonitor;
 
 namespace mozilla {
 enum class StyleGtkThemeFamily : uint8_t;
@@ -162,6 +164,8 @@ class nsLookAndFeel final : public nsXPLookAndFeel {
 
   uint32_t mDBusID = 0;
   RefPtr<GDBusProxy> mDBusSettingsProxy;
+  RefPtr<GFile> mKdeColors;
+  RefPtr<GFileMonitor> mKdeColorsMonitor;
   mozilla::Maybe<ColorScheme> mColorSchemePreference;
   int32_t mCaretBlinkTime = 0;
   int32_t mCaretBlinkCount = -1;