The test case for this bug is attachment 639226 [details] from bug 771043 Steps to reproduce: 1. Open test case in a browser window that is in landscape orientation (the rectangle should be blue) 2. Open Web Console (Ctrl+Shift+K on windows) 3. Open Responsive Design Mode (Ctrl+Shift+M) 4. Press rotate so that the page is in portait orientation (rectangle should be green) 5. Exit Responsive Design Mode (Ctrl+Shift+M) 6. Wait for the transition end event to show 7. Re-enter Responsive Design Mode (Ctrl+Shift+M) 8. Check the console for further transition end events Expected results: At step (8) there should be an additional transitionend event caused by switching to portait orientation and the color of the rectangle should change smoothly from blue to green. Actual results: No addition transitionend events are reported at step (8). The rectangle changes instantly to green (i.e. no transition) (Note that this occurs frequently since MediaQueryList listeners aren't implemented widely and so instead many people setup short transitions triggered by media queries and listen for transitionend events.)