Bug 1699930 - Don't let GTK text scale affect non-native scrollbar sizes. r=mstange

This matches the native theme. We plumb it via LookAndFeel to avoid
having to load GTK settings in child processes.

Differential Revision: https://phabricator.services.mozilla.com/D109275

gfx/thebes/gfxPlatformGtk.cpp

@@ -353,6 +353,9 @@ gfxPlatformFontList* gfxPlatformGtk::CreatePlatformFontList() {
 }
 
 int32_t gfxPlatformGtk::GetFontScaleDPI() {
+  MOZ_ASSERT(XRE_IsParentProcess(),
+             "You can access this via LookAndFeel if you need it in child "
+             "processes");
   if (MOZ_LIKELY(sDPI != 0)) {
     return sDPI;
   }

widget/LookAndFeel.h

@@ -408,6 +408,9 @@ class LookAndFeel {
     // should be added to the calculated caret width.
     CaretAspectRatio,
 
+    // GTK text scale factor.
+    TextScaleFactor,
+
     // Not an ID; used to define the range of valid IDs.  Must be last.
     End,
   };

widget/gtk/nsLookAndFeel.cpp

@@ -830,6 +830,9 @@ nsresult nsLookAndFeel::NativeGetFloat(FloatID aID, float& aResult) {
       EnsureInit();
       aResult = mCaretRatio;
       break;
+    case FloatID::TextScaleFactor:
+      aResult = gfxPlatformGtk::GetFontScaleFactor();
+      break;
     default:
       aResult = -1.0;
       rv = NS_ERROR_FAILURE;

widget/gtk/nsNativeThemeGTK.cpp

@@ -84,8 +84,9 @@ static inline gint GetMonitorScaleFactor(nsPresContext* aPresContext) {
       // value. The computed monitor scale factor needs to be rounded before
       // casting to integer to avoid rounding errors which would lead to
       // returning 0.
-      int monitorScale = int(round(rootWidget->GetDefaultScale().scale /
-                                   gfxPlatformGtk::GetFontScaleFactor()));
+      int monitorScale = int(
+          round(rootWidget->GetDefaultScale().scale /
+                LookAndFeel::GetFloat(LookAndFeel::FloatID::TextScaleFactor)));
       // Monitor scale can be negative if it has not been initialized in the
       // puppet widget yet. We also make sure that we return positive value.
       if (monitorScale < 1) {
@@ -1093,8 +1094,7 @@ NS_IMETHODIMP
 nsNativeThemeGTK::DrawWidgetBackground(gfxContext* aContext, nsIFrame* aFrame,
                                        StyleAppearance aAppearance,
                                        const nsRect& aRect,
-                                       const nsRect& aDirtyRect,
-                                       DrawOverflow) {
+                                       const nsRect& aDirtyRect, DrawOverflow) {
   GtkWidgetState state;
   WidgetNodeType gtkWidgetType;
   GtkTextDirection direction = GetTextDirection(aFrame);

widget/gtk/nsWindow.cpp

@@ -4975,6 +4975,9 @@ nsresult nsWindow::Create(nsIWidget* aParent, nsNativeWidget aNativeParent,
                            G_CALLBACK(settings_changed_cb), this);
     g_signal_connect_after(default_settings, "notify::gtk-xft-dpi",
                            G_CALLBACK(settings_xft_dpi_changed_cb), this);
+    // Text resolution affects system fonts and widget sizes.
+    g_signal_connect_after(default_settings, "notify::resolution",
+                           G_CALLBACK(settings_changed_cb), this);
     // For remote LookAndFeel, to refresh the content processes' copies:
     g_signal_connect_after(default_settings, "notify::gtk-cursor-blink-time",
                            G_CALLBACK(settings_changed_cb), this);

widget/nsNativeBasicTheme.cpp

@@ -174,6 +174,15 @@ void nsNativeBasicTheme::RecomputeScrollbarSizes() {
   } else {
     sHorizontalScrollbarHeight = sVerticalScrollbarWidth = defaultSize;
   }
+  // On GTK, widgets don't account for text scale factor, but that's included
+  // in the usual DPI computations, so we undo that here, just like
+  // GetMonitorScaleFactor does it in nsNativeThemeGTK.
+  float scale =
+      LookAndFeel::GetFloat(LookAndFeel::FloatID::TextScaleFactor, 1.0f);
+  if (scale != 1.0f) {
+    sVerticalScrollbarWidth = float(sVerticalScrollbarWidth) / scale;
+    sHorizontalScrollbarHeight = float(sHorizontalScrollbarHeight) / scale;
+  }
 }
 
 static bool IsScrollbarWidthThin(nsIFrame* aFrame) {
@@ -1599,7 +1608,8 @@ bool nsNativeBasicTheme::CreateWebRenderCommandsForWidget(
     return false;
   }
   WebRenderBackendData data{aBuilder, aResources, aSc, aManager};
-  return DoDrawWidgetBackground(data, aFrame, aAppearance, aRect, DrawOverflow::Yes);
+  return DoDrawWidgetBackground(data, aFrame, aAppearance, aRect,
+                                DrawOverflow::Yes);
 }
 
 static LayoutDeviceRect ToSnappedRect(const nsRect& aRect,