--- a/gfx/gl/GLContextProviderCGL.mm
+++ b/gfx/gl/GLContextProviderCGL.mm
@@ -277,18 +277,17 @@ GLContextProviderCGL::CreateForWindow(ns
     } else {
         attribs = aForceAccelerated ? kAttribs_singleBuffered_accel : kAttribs_singleBuffered;
     }
     NSOpenGLContext* context = CreateWithFormat(attribs);
     if (!context) {
         return nullptr;
     }
 
-    // make the context transparent
-    GLint opaque = 0;
+    GLint opaque = gfxPrefs::CompositorGLContextOpaque();
     [context setValues:&opaque forParameter:NSOpenGLCPSurfaceOpacity];
 
     RefPtr<GLContextCGL> glContext = new GLContextCGL(CreateContextFlags::NONE,
                                                       SurfaceCaps::ForRGBA(), context,
                                                       false);
 
     if (!glContext->Init()) {
         glContext = nullptr;

--- a/gfx/thebes/gfxPrefs.h
+++ b/gfx/thebes/gfxPrefs.h
@@ -434,16 +434,17 @@ private:
 #endif
 #if defined(RELEASE_OR_BETA)
   // "Skip" means this is locked to the default value in beta and release.
   DECL_GFX_PREF(Skip, "gfx.blocklist.all",                     BlocklistAll, int32_t, 0);
 #else
   DECL_GFX_PREF(Once, "gfx.blocklist.all",                     BlocklistAll, int32_t, 0);
 #endif
   DECL_GFX_PREF(Live, "gfx.compositor.clearstate",             CompositorClearState, bool, false);
+  DECL_GFX_PREF(Live, "gfx.compositor.glcontext.opaque",       CompositorGLContextOpaque, bool, false);
   DECL_GFX_PREF(Live, "gfx.canvas.auto_accelerate.min_calls",  CanvasAutoAccelerateMinCalls, int32_t, 4);
   DECL_GFX_PREF(Live, "gfx.canvas.auto_accelerate.min_frames", CanvasAutoAccelerateMinFrames, int32_t, 30);
   DECL_GFX_PREF(Live, "gfx.canvas.auto_accelerate.min_seconds", CanvasAutoAccelerateMinSeconds, float, 5.0f);
   DECL_GFX_PREF(Live, "gfx.canvas.azure.accelerated",          CanvasAzureAccelerated, bool, false);
   DECL_GFX_PREF(Once, "gfx.canvas.azure.accelerated.limit",    CanvasAzureAcceleratedLimit, int32_t, 0);
   // 0x7fff is the maximum supported xlib surface size and is more than enough for canvases.
   DECL_GFX_PREF(Live, "gfx.canvas.max-size",                   MaxCanvasSize, int32_t, 0x7fff);
   DECL_GFX_PREF(Once, "gfx.canvas.skiagl.cache-items",         CanvasSkiaGLCacheItems, int32_t, 256);

--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -917,16 +917,20 @@ pref("gfx.webrender.enabled", true);
 #else
 pref("gfx.webrender.enabled", false);
 #endif
 #ifdef XP_WIN
 pref("gfx.webrender.force-angle", true);
 pref("gfx.webrender.program-binary", true);
 #endif
 
+#ifdef XP_MACOSX
+pref("gfx.compositor.glcontext.opaque", false);
+#endif
+
 pref("gfx.webrender.highlight-painted-layers", false);
 pref("gfx.webrender.blob-images", 1);
 pref("gfx.webrender.hit-test", true);
 
 // WebRender debugging utilities.
 pref("gfx.webrender.debug.texture-cache", false);
 pref("gfx.webrender.debug.render-targets", false);
 pref("gfx.webrender.debug.alpha-primitives", false);

--- a/widget/cocoa/nsChildView.mm
+++ b/widget/cocoa/nsChildView.mm
@@ -3845,18 +3845,19 @@ NSEvent* gLastDragMouseDownEvent = nil;
     return 4.0f;
   return [frameView roundedCornerRadius];
 }
 
 -(void)setGLOpaque:(BOOL)aOpaque
 {
   CGLLockContext((CGLContextObj)[mGLContext CGLContextObj]);
   // Make the context opaque for fullscreen (since it performs better), and transparent
-  // for windowed (since we need it for rounded corners).
-  GLint opaque = aOpaque ? 1 : 0;
+  // for windowed (since we need it for rounded corners), but allow overriding
+  // it to opaque for testing purposes, even if that breaks the rounded corners.
+  GLint opaque = aOpaque || gfxPrefs::CompositorGLContextOpaque();
   [mGLContext setValues:&opaque forParameter:NSOpenGLCPSurfaceOpacity];
   CGLUnlockContext((CGLContextObj)[mGLContext CGLContextObj]);
 }
 
 // Accelerated windows have two NSSurfaces:
 //  (1) The window's pixel buffer in the back and
 //  (2) the OpenGL view in the front.
 // These two surfaces are composited by the window manager. Drawing into the