Bug 1554850 - handle fractional DPI change on Linux. r=karlt

On Linux, Firefox is listening on notify::scale-factor to detect DPI
change. However, scale-factor is an int and on the lower-end side of
the DPI scale, some devices are using fractional scale factors encoded
into Xft/DPI setting. Changing from ×1 to ×1.5 scale is therefore
undetected.

The proposed change is two-folds:

 - remove use of a cached sDPI value and rely on GTK being the cache

 - listening on notify::gtk-xft-dpi to trigger a DPI change

What is missing:

 - performance evaluation of not caching sDPI (on a 10s session
   loading 2 pages, there is an "overhead" of 6ms on my setup, nothing
   visible from my point of view)

 - when changing Xft/DPI and scale, the change is done twice, this
   seems harmless

Differential Revision: https://phabricator.services.mozilla.com/D92095

gfx/thebes/gfxPlatformGtk.cpp

@@ -245,22 +245,15 @@ gfxPlatformFontList* gfxPlatformGtk::CreatePlatformFontList() {
   return nullptr;
 }
 
-// FIXME(emilio, bug 1554850): This should be invalidated somehow, right now
-// requires a restart.
-static int32_t sDPI = 0;
-
 int32_t gfxPlatformGtk::GetFontScaleDPI() {
-  if (MOZ_UNLIKELY(!sDPI)) {
-    // Make sure init is run so we have a resolution
-    GdkScreen* screen = gdk_screen_get_default();
-    gtk_settings_get_for_screen(screen);
-    sDPI = int32_t(round(gdk_screen_get_resolution(screen)));
-    if (sDPI <= 0) {
-      // Fall back to something sane
-      sDPI = 96;
-    }
+  GdkScreen* screen = gdk_screen_get_default();
+  gtk_settings_get_for_screen(screen);
+  int32_t dpi = int32_t(round(gdk_screen_get_resolution(screen)));
+  if (dpi <= 0) {
+    // Fall back to something sane
+    dpi = 96;
   }
-  return sDPI;
+  return dpi;
 }
 
 double gfxPlatformGtk::GetFontScaleFactor() {

widget/gtk/nsWindow.cpp

@@ -218,6 +218,8 @@ static gboolean window_state_event_cb(GtkWidget* widget,
                                       GdkEventWindowState* event);
 static void settings_changed_cb(GtkSettings* settings, GParamSpec* pspec,
                                 nsWindow* data);
+static void settings_xft_dpi_changed_cb(GtkSettings* settings,
+                                        GParamSpec* pspec, nsWindow* data);
 static void check_resize_cb(GtkContainer* container, gpointer user_data);
 static void screen_composited_changed_cb(GdkScreen* screen, gpointer user_data);
 static void widget_composited_changed_cb(GtkWidget* widget, gpointer user_data);
@@ -715,8 +717,7 @@ void nsWindow::Destroy() {
 
   ClearCachedResources();
 
-  g_signal_handlers_disconnect_by_func(
-      gtk_settings_get_default(), FuncToGpointer(settings_changed_cb), this);
+  g_signal_handlers_disconnect_by_data(gtk_settings_get_default(), this);
 
   nsIRollupListener* rollupListener = nsBaseWidget::GetActiveRollupListener();
   if (rollupListener) {
@@ -4690,6 +4691,8 @@ nsresult nsWindow::Create(nsIWidget* aParent, nsNativeWidget aNativeParent,
                            G_CALLBACK(settings_changed_cb), this);
     g_signal_connect_after(default_settings, "notify::gtk-decoration-layout",
                            G_CALLBACK(settings_changed_cb), this);
+    g_signal_connect_after(default_settings, "notify::gtk-xft-dpi",
+                           G_CALLBACK(settings_xft_dpi_changed_cb), this);
   }
 
   if (mContainer) {
@@ -6821,6 +6824,16 @@ static void settings_changed_cb(GtkSettings* settings, GParamSpec* pspec,
   window->ThemeChanged();
 }
 
+static void settings_xft_dpi_changed_cb(GtkSettings* gtk_settings,
+                                        GParamSpec* pspec, nsWindow* data) {
+  RefPtr<nsWindow> window = data;
+  window->OnDPIChanged();
+  // Even though the window size in screen pixels has not changed,
+  // nsViewManager stores the dimensions in app units.
+  // DispatchResized() updates those.
+  window->DispatchResized();
+}
+
 static void check_resize_cb(GtkContainer* container, gpointer user_data) {
   RefPtr<nsWindow> window = get_window_for_gtk_widget(GTK_WIDGET(container));
   if (!window) {

widget/gtk/nsWindow.h

@@ -271,6 +271,7 @@ class nsWindow final : public nsBaseWidget {
   void OnCheckResize(void);
   void OnCompositedChanged(void);
   void OnScaleChanged(GtkAllocation* aAllocation);
+  void DispatchResized();
 
 #ifdef MOZ_X11
   Window mOldFocusWindow;
@@ -429,7 +430,6 @@ class nsWindow final : public nsBaseWidget {
   // event handling code
   void DispatchActivateEvent(void);
   void DispatchDeactivateEvent(void);
-  void DispatchResized();
   void MaybeDispatchResized();
 
   virtual void RegisterTouchWindow() override;