Bug 1580117 - Pause composition for minimized windows on macOS. r=mst…

…ange

This pauses composition when a window becomes minimized.

This is particularly important for platforms like macOS, where minimized /
hidden windows historically would remain compositing, potentially on every vsync
if the hidden window contained animations. This should lead to a nice power
savings by skipping this unnecessary work.

This change affects both the WebRender and legacy compositor paths.

As bug 1580117 is believed to be a macOS specific issue, this is currently
implemented for macOS only, but could in the future be generalized to other
platforms as well.

Differential Revision: https://phabricator.services.mozilla.com/D85954

modules/libpref/init/StaticPrefList.yaml

@@ -10015,6 +10015,12 @@
   value: true
   mirror: always
 
+# Whether to pause the compositor when a native window is minimized.
+- name: widget.pause-compositor-when-minimized
+  type: bool
+  value: true
+  mirror: always
+
 #ifdef MOZ_WAYLAND
 #ifdef NIGHTLY_BUILD
 # Keep those pref hidden on non-nightly builds to avoid people accidentally

widget/cocoa/nsCocoaWindow.h

@@ -340,6 +340,9 @@ class nsCocoaWindow final : public nsBaseWidget, public nsPIWidgetCocoa {
 
   bool InFullScreenMode() const { return mInFullScreenMode; }
 
+  void PauseCompositor();
+  void ResumeCompositor();
+
  protected:
   virtual ~nsCocoaWindow();
 

widget/cocoa/nsCocoaWindow.mm

@@ -34,6 +34,8 @@
 #include "nsIScreenManager.h"
 #include "nsIWidgetListener.h"
 #include "VibrancyManager.h"
+#include "nsPresContext.h"
+#include "nsDocShell.h"
 
 #include "gfxPlatform.h"
 #include "qcms.h"
@@ -44,6 +46,7 @@
 #include "mozilla/StaticPrefs_gfx.h"
 #include "mozilla/StaticPrefs_widget.h"
 #include "mozilla/PresShell.h"
+#include "mozilla/layers/CompositorBridgeChild.h"
 #include <algorithm>
 
 namespace mozilla {
@@ -2072,6 +2075,14 @@ static nsSizeMode GetWindowSizeMode(NSWindow* aWindow, bool aFullScreen) {
   if (mWidgetListener) {
     mWidgetListener->SizeModeChanged(newMode);
   }
+
+  if (StaticPrefs::widget_pause_compositor_when_minimized()) {
+    if (newMode == nsSizeMode_Minimized) {
+      PauseCompositor();
+    } else {
+      ResumeCompositor();
+    }
+  }
 }
 
 void nsCocoaWindow::DispatchOcclusionEvent() {
@@ -2105,6 +2116,72 @@ static nsSizeMode GetWindowSizeMode(NSWindow* aWindow, bool aFullScreen) {
   NS_OBJC_END_TRY_ABORT_BLOCK;
 }
 
+void nsCocoaWindow::PauseCompositor() {
+  nsIWidget* mainChildView = static_cast<nsIWidget*>([[mWindow mainChildView] widget]);
+  if (!mainChildView) {
+    return;
+  }
+  CompositorBridgeChild* remoteRenderer = mainChildView->GetRemoteRenderer();
+  if (!remoteRenderer) {
+    return;
+  }
+  remoteRenderer->SendPause();
+
+  // Now that the compositor has paused, we also try to mark the browser window
+  // docshell inactive to stop any animations. This does not affect docshells
+  // for browsers in other processes, but browser UI code should be managing
+  // their active state appropriately.
+  if (!mWidgetListener) {
+    return;
+  }
+  PresShell* presShell = mWidgetListener->GetPresShell();
+  if (!presShell) {
+    return;
+  }
+  nsPresContext* presContext = presShell->GetPresContext();
+  if (!presContext) {
+    return;
+  }
+  nsDocShell* docShell = presContext->GetDocShell();
+  if (!docShell) {
+    return;
+  }
+  docShell->SetIsActive(false);
+}
+
+void nsCocoaWindow::ResumeCompositor() {
+  nsIWidget* mainChildView = static_cast<nsIWidget*>([[mWindow mainChildView] widget]);
+  if (!mainChildView) {
+    return;
+  }
+  CompositorBridgeChild* remoteRenderer = mainChildView->GetRemoteRenderer();
+  if (!remoteRenderer) {
+    return;
+  }
+  remoteRenderer->SendResume();
+
+  // Now that the compositor has resumed, we also try to mark the browser window
+  // docshell active to restart any animations. This does not affect docshells
+  // for browsers in other processes, but browser UI code should be managing
+  // their active state appropriately.
+  if (!mWidgetListener) {
+    return;
+  }
+  PresShell* presShell = mWidgetListener->GetPresShell();
+  if (!presShell) {
+    return;
+  }
+  nsPresContext* presContext = presShell->GetPresContext();
+  if (!presContext) {
+    return;
+  }
+  nsDocShell* docShell = presContext->GetDocShell();
+  if (!docShell) {
+    return;
+  }
+  docShell->SetIsActive(true);
+}
+
 void nsCocoaWindow::SetMenuBar(nsMenuBarX* aMenuBar) {
   if (mMenuBar) mMenuBar->SetParent(nullptr);
   if (!mWindow) {