Merge "Ensure DockedStackDividerController sets a dim for one stack at a time."

                : null;

        final TaskStack dockedStack = mDisplayContent.getSplitScreenPrimaryStack();

        boolean visibleAndValid = visible && stack != null && dockedStack != null;

        // Ensure an old dim that was shown for the docked stack divider is removed so we don't end

        // up with dim layers that can no longer be removed.

        if (mDimmedStack != null && mDimmedStack != stack) {

            mDimmedStack.stopDimming();

            mDimmedStack = null;

        }

        if (visibleAndValid) {

            mDimmedStack = stack;

            stack.dim(alpha);