Respect GDK_SCALE and GDK_DPI_SCALE when calculating scale factor

chrome/browser/ui/libgtkui/gtk_ui.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/ui/libgtkui/gtk_ui.h"
 
 #include <X11/Xcursor/Xcursor.h>
 #include <dlfcn.h>
 #include <math.h>
 #include <pango/pango.h>
@@ skipping 294 matching lines @@
     LOG(WARNING) << "Unexpected gtk-xft-rgba \"" << rgba << "\"";
     params.subpixel_rendering = gfx::FontRenderParams::SUBPIXEL_RENDERING_NONE;
   }
 
   g_free(hint_style);
   g_free(rgba);
 
   return params;
 }
 
-float GtkDpiToScaleFactor(int dpi) {
-  // GTK multiplies the DPI by 1024 before storing it.
-  return dpi / (1024 * kDefaultDPI);
-}
-
-gint GetGdkScreenSettingInt(const char* setting_name) {
-  GValue value = G_VALUE_INIT;
-  g_value_init(&value, G_TYPE_INT);
-  if (!gdk_screen_get_setting(gdk_screen_get_default(), setting_name, &value))
-    return -1;
-  return g_value_get_int(&value);
-}
-
-float GetScaleFromGdkScreenSettings() {
-  gint window_scale = GetGdkScreenSettingInt("gdk-window-scaling-factor");
-  if (window_scale <= 0)
-    return -1;
-  gint font_dpi = GetGdkScreenSettingInt("gdk-unscaled-dpi");
-  if (font_dpi <= 0)
-    return -1;
-  return window_scale * GtkDpiToScaleFactor(font_dpi);
-}
-
-float GetScaleFromXftDPI() {
-  GtkSettings* gtk_settings = gtk_settings_get_default();
-  CHECK(gtk_settings);
-  gint gtk_dpi = -1;
-  g_object_get(gtk_settings, "gtk-xft-dpi", &gtk_dpi, nullptr);
-  if (gtk_dpi <= 0)
-    return -1;
-  return GtkDpiToScaleFactor(gtk_dpi);
-}
-
 float GetRawDeviceScaleFactor() {
   if (display::Display::HasForceDeviceScaleFactor())
     return display::Display::GetForcedDeviceScaleFactor();
 
-  float scale = GetScaleFromGdkScreenSettings();
-  if (scale > 0)
-    return scale;
-
-  scale = GetScaleFromXftDPI();
-  if (scale > 0)
-    return scale;
-
-  return 1;
+  GdkScreen* screen = gdk_screen_get_default();
+  gint scale = gdk_screen_get_monitor_scale_factor(
+      screen, gdk_screen_get_primary_monitor(screen));
+  gdouble resolution = gdk_screen_get_resolution(screen);
+  return resolution <= 0 ? scale : resolution * scale / kDefaultDPI;
 }
 
 views::LinuxUI::NonClientMiddleClickAction GetDefaultMiddleClickAction() {
   std::unique_ptr<base::Environment> env(base::Environment::Create());
   switch (base::nix::GetDesktopEnvironment(env.get())) {
     case base::nix::DESKTOP_ENVIRONMENT_KDE4:
     case base::nix::DESKTOP_ENVIRONMENT_KDE5:
       // Starting with KDE 4.4, windows' titlebars can be dragged with the
       // middle mouse button to create tab groups. We don't support that in
       // Chrome, but at least avoid lowering windows in response to middle
@@ skipping 713 matching lines @@
 
 float GtkUi::GetDeviceScaleFactor() const {
   return device_scale_factor_;
 }
 
 }  // namespace libgtkui
 
 views::LinuxUI* BuildGtkUi() {
   return new libgtkui::GtkUi;
 }