Bug 1400317 - Use gfxVars to mirror DWM status to content processes r…

…=jrmuizel

The only caveat here is that gfxVars are not atomic, but multiple threads can
query DWM status. To solve this, changes to the var are mirrored into an atomic
and that is read instead.

DWM status changes are indicated by Windows via a window message. We use that
window message to cause the update to propagate

Differential Revision: https://phabricator.services.mozilla.com/D73743

gfx/config/gfxVars.h

@@ -61,7 +61,8 @@ class gfxVarReceiver;
   _(RemoteCanvasEnabled, bool, false)                              \
   _(UseDoubleBufferingWithCompositor, bool, false)                 \
   _(UseGLSwizzle, bool, true)                                      \
-  _(AllowSacrificingSubpixelAA, bool, false)
+  _(AllowSacrificingSubpixelAA, bool, false)                       \
+  _(DwmCompositionEnabled, bool, true)
 
 /* Add new entries above this line. */
 

gfx/thebes/gfxWindowsPlatform.cpp

@@ -281,7 +281,9 @@ class D3DSharedTexturesReporter final : public nsIMemoryReporter {
 
 NS_IMPL_ISUPPORTS(D3DSharedTexturesReporter, nsIMemoryReporter)
 
-gfxWindowsPlatform::gfxWindowsPlatform() : mRenderMode(RENDER_GDI) {
+gfxWindowsPlatform::gfxWindowsPlatform()
+    : mRenderMode(RENDER_GDI),
+      mDwmCompositionStatus(DwmCompositionStatus::Unknown) {
   /*
    * Initialize COM
    */
@@ -335,6 +337,27 @@ void gfxWindowsPlatform::InitAcceleration() {
   gfxVars::SetSystemTextQualityListener(
       gfxDWriteFont::SystemTextQualityChanged);
 
+  if (XRE_IsParentProcess()) {
+    BOOL dwmEnabled = FALSE;
+    if (FAILED(::DwmIsCompositionEnabled(&dwmEnabled)) || !dwmEnabled) {
+      gfxVars::SetDwmCompositionEnabled(false);
+    } else {
+      gfxVars::SetDwmCompositionEnabled(true);
+    }
+  }
+
+  // gfxVars are not atomic, but multiple threads can query DWM status
+  // Therefore, mirror value into an atomic
+  mDwmCompositionStatus = gfxVars::DwmCompositionEnabled()
+                              ? DwmCompositionStatus::Enabled
+                              : DwmCompositionStatus::Disabled;
+
+  gfxVars::SetDwmCompositionEnabledListener([this] {
+    this->mDwmCompositionStatus = gfxVars::DwmCompositionEnabled()
+                                      ? DwmCompositionStatus::Enabled
+                                      : DwmCompositionStatus::Disabled;
+  });
+
   // CanUseHardwareVideoDecoding depends on DeviceManagerDx state,
   // so update the cached value now.
   UpdateCanUseHardwareVideoDecoding();
@@ -1610,13 +1633,9 @@ bool gfxWindowsPlatform::InitGPUProcessSupport() {
 }
 
 bool gfxWindowsPlatform::DwmCompositionEnabled() {
-  BOOL dwmEnabled = false;
-
-  if (FAILED(DwmIsCompositionEnabled(&dwmEnabled))) {
-    return false;
-  }
+  MOZ_RELEASE_ASSERT(mDwmCompositionStatus != DwmCompositionStatus::Unknown);
 
-  return dwmEnabled;
+  return mDwmCompositionStatus == DwmCompositionStatus::Enabled;
 }
 
 class D3DVsyncSource final : public VsyncSource {

gfx/thebes/gfxWindowsPlatform.h

@@ -226,6 +226,12 @@ class gfxWindowsPlatform final : public gfxPlatform {
   RenderMode mRenderMode;
 
  private:
+  enum class DwmCompositionStatus : uint32_t {
+    Unknown,
+    Disabled,
+    Enabled,
+  };
+
   void Init();
   void InitAcceleration() override;
   void InitWebRenderConfig() override;
@@ -254,6 +260,9 @@ class gfxWindowsPlatform final : public gfxPlatform {
   RefPtr<mozilla::layers::ReadbackManagerD3D11> mD3D11ReadbackManager;
   bool mInitializedDevices = false;
 
+  mozilla::Atomic<DwmCompositionStatus, mozilla::ReleaseAcquire>
+      mDwmCompositionStatus;
+
   // Cached contents of the output color profile file
   nsTArray<uint8_t> mCachedOutputColorProfile;
 };

widget/windows/nsWindow.cpp

@@ -5994,6 +5994,17 @@ bool nsWindow::ProcessMessage(UINT msg, WPARAM& wParam, LPARAM& lParam,
     } break;
 
     case WM_DWMCOMPOSITIONCHANGED:
+      // Every window will get this message, but gfxVars only broadcasts
+      // updates when the value actually changes
+      if (XRE_IsParentProcess()) {
+        BOOL dwmEnabled = FALSE;
+        if (FAILED(::DwmIsCompositionEnabled(&dwmEnabled)) || !dwmEnabled) {
+          gfxVars::SetDwmCompositionEnabled(false);
+        } else {
+          gfxVars::SetDwmCompositionEnabled(true);
+        }
+      }
+
       UpdateNonClientMargins();
       BroadcastMsg(mWnd, WM_DWMCOMPOSITIONCHANGED);
       NotifyThemeChanged();