Bug 1256731 - patch 2 - Notify front-end code when screen resolution changes on Windows, so that TabsInTitlebar code can refresh its computations. r=gijs,emk

browser/base/content/browser-tabsintitlebar.js

@@ -35,6 +35,8 @@ var TabsInTitlebar = {
     };
     CustomizableUI.addListener(this);
 
+    addEventListener("resolutionchange", this, false);
+
     this._initialized = true;
   },
 
@@ -65,6 +67,12 @@ var TabsInTitlebar = {
       this._readPref();
   },
 
+  handleEvent: function (aEvent) {
+    if (aEvent.type == "resolutionchange" && aEvent.target == window) {
+      this._update(true);
+    }
+  },
+
   _onMenuMutate: function (aMutations) {
     for (let mutation of aMutations) {
       if (mutation.attributeName == "inactive" ||
@@ -241,6 +249,7 @@ var TabsInTitlebar = {
 
   uninit: function () {
     this._initialized = false;
+    removeEventListener("resolutionchange", this);
     Services.prefs.removeObserver(this._prefName, this);
     this._menuObserver.disconnect();
     CustomizableUI.removeListener(this);

widget/nsIWidgetListener.cpp

@@ -54,6 +54,11 @@ nsIWidgetListener::SizeModeChanged(nsSizeMode aSizeMode)
 {
 }
 
+void
+nsIWidgetListener::UIResolutionChanged()
+{
+}
+
 void
 nsIWidgetListener::FullscreenChanged(bool aInFullscreen)
 {

widget/nsIWidgetListener.h

@@ -79,6 +79,12 @@ public:
    */
   virtual void SizeModeChanged(nsSizeMode aSizeMode);
 
+  /**
+   * Called when the DPI (device resolution scaling factor) is changed,
+   * such that UI elements may need to be rescaled.
+   */
+  virtual void UIResolutionChanged();
+
   /**
    * Called when the z-order of the window is changed. Returns true if the
    * notification was handled. aPlacement indicates the new z order. If

widget/windows/nsWindowBase.cpp

@@ -112,6 +112,7 @@ void nsWindowBase::ChangedDPI()
     if (presShell) {
       presShell->BackingScaleFactorChanged();
     }
+    mWidgetListener->UIResolutionChanged();
   }
 }
 

xpfe/appshell/nsWebShellWindow.cpp

@@ -377,6 +377,17 @@ nsWebShellWindow::SizeModeChanged(nsSizeMode sizeMode)
   // then need to be different.
 }
 
+void
+nsWebShellWindow::UIResolutionChanged()
+{
+  nsCOMPtr<nsPIDOMWindowOuter> ourWindow =
+    mDocShell ? mDocShell->GetWindow() : nullptr;
+  if (ourWindow) {
+    MOZ_ASSERT(ourWindow->IsOuterWindow());
+    ourWindow->DispatchCustomEvent(NS_LITERAL_STRING("resolutionchange"));
+  }
+}
+
 void
 nsWebShellWindow::FullscreenChanged(bool aInFullscreen)
 {

xpfe/appshell/nsWebShellWindow.h

@@ -56,6 +56,7 @@ public:
   virtual bool WindowResized(nsIWidget* aWidget, int32_t aWidth, int32_t aHeight) override;
   virtual bool RequestWindowClose(nsIWidget* aWidget) override;
   virtual void SizeModeChanged(nsSizeMode sizeMode) override;
+  virtual void UIResolutionChanged() override;
   virtual void FullscreenChanged(bool aInFullscreen) override;
   virtual void OSToolbarButtonPressed() override;
   virtual bool ZLevelChanged(bool aImmediate, nsWindowZ *aPlacement,