Glitches are displayed when clicking the "Delete" and "Undo" buttons

[SoftVision-CarmenFat]

[Affected versions]:

• Release 58.0 and up
• Snooze Tabs 1.0.21

[Affected Platforms]:

• All Windows

[Prerequisites]:

• Have a Firefox profile with the latest Snooze Tabs add-on version (https://669-15314951-gh.circle-artifacts.com/0/tmp/circle-artifacts.kZdit8p/addon.xpi) loaded as a temporary add-on.
• Have at least one snoozed tab in the "Manage Snoozed Tabs" panel.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisite and navigate to a website.
2. Click the "Snooze Tabs" button from the toolbar.
3. Click the "Manage Snoozed Tabs" button.
4. Click on the "Delete" button and move the mouse cursor away from the button.
5. Click on the "Undo" button and move the mouse cursor away from the button.
6. Observe the UI of the buttons. Bonus:
7. Click the "Undo" button and move the mouse cursor away from the button.

[Expected result]: 6. No UI glitches are displayed. 7. No UI glitches are displayed.

[Actual result]: 6. There are glitches displayed - usually the top part of the buttons remains displayed for a second before hiding. 7. A grey glitch is displayed.

[Notes]:

• Attached a screen recording of the issue: https://goo.gl/E7TLEv.

[lmorchard]

@johngruen Were you still looking into this?