diff --git a/gfx/webrender_bindings/RenderCompositorSWGL.cpp b/gfx/webrender_bindings/RenderCompositorSWGL.cpp
index 1c24407b3493..ec5be61f4d10 100644
--- a/gfx/webrender_bindings/RenderCompositorSWGL.cpp
+++ b/gfx/webrender_bindings/RenderCompositorSWGL.cpp
@@ -10,6 +10,8 @@
 #include "mozilla/widget/CompositorWidget.h"
 
 namespace mozilla {
+using namespace gfx;
+
 namespace wr {
 
 /* static */
@@ -53,7 +55,8 @@ bool RenderCompositorSWGL::BeginFrame() {
   return true;
 }
 
-bool RenderCompositorSWGL::AllocateMappedBuffer() {
+bool RenderCompositorSWGL::AllocateMappedBuffer(
+    const wr::DeviceIntRect* aOpaqueRects, size_t aNumOpaqueRects) {
   // Request a new draw target to use from the widget...
   MOZ_ASSERT(!mDT);
   layers::BufferMode bufferMode = layers::BufferMode::BUFFERED;
@@ -113,6 +116,25 @@ bool RenderCompositorSWGL::AllocateMappedBuffer() {
   MOZ_ASSERT(mMappedData != nullptr && mMappedStride > 0);
   wr_swgl_init_default_framebuffer(mContext, bounds.x, bounds.y, bounds.width,
                                    bounds.height, mMappedStride, mMappedData);
+
+  LayoutDeviceIntRegion opaque;
+  for (size_t i = 0; i < aNumOpaqueRects; i++) {
+    const auto& rect = aOpaqueRects[i];
+    opaque.OrWith(LayoutDeviceIntRect(rect.origin.x, rect.origin.y,
+                                      rect.size.width, rect.size.height));
+  }
+
+  RefPtr<DrawTarget> dt = gfx::Factory::CreateDrawTargetForData(
+      BackendType::SKIA, mMappedData, bounds.Size().ToUnknownSize(),
+      mMappedStride, SurfaceFormat::B8G8R8A8, false);
+
+  LayoutDeviceIntRegion clear;
+  clear.Sub(mRegion, opaque);
+  for (auto iter = clear.RectIter(); !iter.Done(); iter.Next()) {
+    dt->ClearRect(
+        IntRectToRect((iter.Get() - bounds.TopLeft()).ToUnknownRect()));
+  }
+
   return true;
 }
 
@@ -140,7 +162,7 @@ void RenderCompositorSWGL::StartCompositing(
   // Now that the dirty rects have been supplied and the composition region
   // is known, allocate and install a framebuffer encompassing the composition
   // region.
-  if (!AllocateMappedBuffer()) {
+  if (!AllocateMappedBuffer(aOpaqueRects, aNumOpaqueRects)) {
     gfxCriticalNote
         << "RenderCompositorSWGL failed mapping default framebuffer";
     // If allocation of the mapped default framebuffer failed, then just install
diff --git a/gfx/webrender_bindings/RenderCompositorSWGL.h b/gfx/webrender_bindings/RenderCompositorSWGL.h
index fa78d3021491..b0f53520e05d 100644
--- a/gfx/webrender_bindings/RenderCompositorSWGL.h
+++ b/gfx/webrender_bindings/RenderCompositorSWGL.h
@@ -67,7 +67,8 @@ class RenderCompositorSWGL : public RenderCompositor {
 
   void ClearMappedBuffer();
 
-  bool AllocateMappedBuffer();
+  bool AllocateMappedBuffer(const wr::DeviceIntRect* aOpaqueRects,
+                            size_t aNumOpaqueRects);
 
   void CommitMappedBuffer(bool aDirty = true);
 };