Bug 901382. Don't fall back to basic OMTC. r=mattwoodrow

modules/libpref/src/init/all.js

@@ -4091,6 +4091,10 @@ pref("layers.offmainthreadcomposition.enabled", false);
 // same effect as layers.offmainthreadcomposition.enabled, but specifically for
 // use with tests.
 pref("layers.offmainthreadcomposition.testing.enabled", false);
+
+// whether to allow use of the basic compositor
+pref("layers.offmainthreadcomposition.force-basic", false);
+
 // Whether to animate simple opacity and transforms on the compositor
 pref("layers.offmainthreadcomposition.async-animations", false);
 // Whether to prefer normal memory over shared memory. Ignored with cross-process compositing

widget/cocoa/nsChildView.mm

@@ -1534,13 +1534,6 @@ nsChildView::ComputeShouldAccelerate(bool aDefault)
 bool
 nsChildView::ShouldUseOffMainThreadCompositing()
 {
-  // When acceleration is off, default to false, but allow force-enabling
-  // using the layers.offmainthreadcomposition.prefer-basic pref.
-  if (!ComputeShouldAccelerate(mUseLayersAcceleration) &&
-      !Preferences::GetBool("layers.offmainthreadcomposition.prefer-basic", false)) {
-    return false;
-  }
-
   // Don't use OMTC (which requires OpenGL) for transparent windows or for
   // popup windows.
   if (!mView || ![[mView window] isOpaque] ||

widget/xpwidgets/nsBaseWidget.cpp

@@ -947,9 +947,18 @@ void nsBaseWidget::CreateCompositor(int aWidth, int aHeight)
   PLayerTransactionChild* shadowManager;
   mozilla::layers::LayersBackend backendHint = GetPreferredCompositorBackend();
 
-  bool success;
-  shadowManager = mCompositorChild->SendPLayerTransactionConstructor(
-    backendHint, 0, &textureFactoryIdentifier, &success);
+  if (backendHint == LAYERS_BASIC &&
+      !Preferences::GetBool("layers.offmainthreadcomposition.force-basic", false) &&
+      !Preferences::GetBool("browser.tabs.remote", false)) {
+    // basic compositor is not stable enough for regular use
+    backendHint = LAYERS_NONE;
+  }
+
+  bool success = false;
+  if (backendHint) {
+    shadowManager = mCompositorChild->SendPLayerTransactionConstructor(
+      backendHint, 0, &textureFactoryIdentifier, &success);
+  }
 
   if (success) {
     ShadowLayerForwarder* lf = lm->AsShadowForwarder();