3 files changed, 14 insertions, 0 deletions
diff --git a/widget/windows/WinCompositorWidget.cpp b/widget/windows/WinCompositorWidget.cpp
index f660bd019..99ce67573 100644
--- a/widget/windows/WinCompositorWidget.cpp
+++ b/widget/windows/WinCompositorWidget.cpp
@@ -22,6 +22,7 @@ using namespace mozilla::gfx;
 WinCompositorWidget::WinCompositorWidget(const CompositorWidgetInitData& aInitData)
  : mWidgetKey(aInitData.widgetKey()),
    mWnd(reinterpret_cast<HWND>(aInitData.hWnd())),
+   mTransparentSurfaceLock("mTransparentSurfaceLock"),
    mTransparencyMode(static_cast<nsTransparencyMode>(aInitData.transparencyMode())),
    mMemoryDC(nullptr),
    mCompositeDC(nullptr),
@@ -39,6 +40,7 @@ WinCompositorWidget::WinCompositorWidget(const CompositorWidgetInitData& aInitDa
 void
 WinCompositorWidget::OnDestroyWindow()
 {
+  MutexAutoLock lock(mTransparentSurfaceLock);
   mTransparentSurface = nullptr;
   mMemoryDC = nullptr;
 }
@@ -75,6 +77,8 @@ WinCompositorWidget::GetClientSize()
 already_AddRefed<gfx::DrawTarget>
 WinCompositorWidget::StartRemoteDrawing()
 {
+  MutexAutoLock lock(mTransparentSurfaceLock);
+
   MOZ_ASSERT(!mCompositeDC);
 
   RefPtr<gfxASurface> surf;
@@ -229,6 +233,7 @@ WinCompositorWidget::LeavePresentLock()
 RefPtr<gfxASurface>
 WinCompositorWidget::EnsureTransparentSurface()
 {
+  mTransparentSurfaceLock.AssertCurrentThreadOwns();
   MOZ_ASSERT(mTransparencyMode == eTransparencyTransparent);
 
   IntSize size = GetClientSize().ToUnknownSize();
@@ -245,6 +250,7 @@ WinCompositorWidget::EnsureTransparentSurface()
 void
 WinCompositorWidget::CreateTransparentSurface(const gfx::IntSize& aSize)
 {
+  mTransparentSurfaceLock.AssertCurrentThreadOwns();
   MOZ_ASSERT(!mTransparentSurface && !mMemoryDC);
   RefPtr<gfxWindowsSurface> surface = new gfxWindowsSurface(aSize, SurfaceFormat::A8R8G8B8_UINT32);
   mTransparentSurface = surface;
@@ -254,6 +260,7 @@ WinCompositorWidget::CreateTransparentSurface(const gfx::IntSize& aSize)
 void
 WinCompositorWidget::UpdateTransparency(nsTransparencyMode aMode)
 {
+  MutexAutoLock lock(mTransparentSurfaceLock);
   if (mTransparencyMode == aMode) {
     return;
   }
@@ -270,6 +277,7 @@ WinCompositorWidget::UpdateTransparency(nsTransparencyMode aMode)
 void
 WinCompositorWidget::ClearTransparentWindow()
 {
+  MutexAutoLock lock(mTransparentSurfaceLock);
   if (!mTransparentSurface) {
     return;
   }
diff --git a/widget/windows/WinCompositorWidget.h b/widget/windows/WinCompositorWidget.h
index 9661cab45..1689a8641 100644
--- a/widget/windows/WinCompositorWidget.h
+++ b/widget/windows/WinCompositorWidget.h
@@ -10,6 +10,7 @@
 #include "gfxASurface.h"
 #include "mozilla/gfx/CriticalSection.h"
 #include "mozilla/gfx/Point.h"
+#include "mozilla/Mutex.h"
 #include "nsIWidget.h"
 
 class nsWindow;
@@ -83,6 +84,8 @@ public:
     return mWnd;
   }
 
+  mozilla::Mutex& GetTransparentSurfaceLock() { return mTransparentSurfaceLock; }
+
 private:
   HDC GetWindowSurface();
   void FreeWindowSurface(HDC dc);
@@ -95,6 +98,7 @@ private:
   gfx::CriticalSection mPresentLock;
 
   // Transparency handling.
+  mozilla::Mutex mTransparentSurfaceLock;
   nsTransparencyMode mTransparencyMode;
   RefPtr<gfxASurface> mTransparentSurface;
   HDC mMemoryDC;
diff --git a/widget/windows/nsWindowGfx.cpp b/widget/windows/nsWindowGfx.cpp
index a88631f89..9b303a0f2 100644
--- a/widget/windows/nsWindowGfx.cpp
+++ b/widget/windows/nsWindowGfx.cpp
@@ -320,6 +320,8 @@ bool nsWindow::OnPaint(HDC aDC, uint32_t aNestingLevel)
 #if defined(MOZ_XUL)
           // don't support transparency for non-GDI rendering, for now
           if (eTransparencyTransparent == mTransparencyMode) {
+            // This mutex needs to be held when EnsureTransparentSurface is called.
+            MutexAutoLock lock(mBasicLayersSurface->GetTransparentSurfaceLock());
             targetSurface = mBasicLayersSurface->EnsureTransparentSurface();
           }
 #endif