An empty space is displayed on the right side of the panel if opening Snooze Tabs from the "More tools..." button

[SoftVision-CarmenFat]

[Affected versions]:

• Firefox 58.0.2 and above
• Snooze Tabs 1.1.0

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Have a Firefox profile with the latest Snooze Tabs add-on version (https://github.com/bwinton/SnoozeTabs/releases/tag/1.1.0) loaded as a temporary add-on.
• Firefox browser is resized until the "More tools..." is displayed.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites and navigate to a website (e.g. "www.wikipedia.org").
2. Click the "More tools" button.
3. Click the "Snooze tab until..." button.
4. Observe the way the panel is displayed.

[Expected result]:

• The panel is correctly displayed.

[Actual results]:

• There is an empty space on the right side of the panel.

[Notes]:

• This is also reproducible with the Test Pilot version of Snooze Tabs.
• The issue is also reproducible on macOS, even though the empty space is way smaller than on Windows.
• Attached a screencast of the issue: