"Move tabs to the top" button wrongly expands when hovering it on Linux

[SoftVision-CarmenFat]

[Affected versions]:

• Firefox 53.0 and above

[Affected Platforms]:

• All Linux

[Prerequisites]:

• Tab Center was already installed on a new profile.

[Steps to reproduce]:

1. Open Firefox browser with the profile from prerequisites.
2. Hover the "Move tabs to the top" button from the upper side of the Sidebar.
3. Observe the way the button reacts when hovered.

[Expected result]:

• Grey highlight state of the button is displayed when hovering the button.

[Actual result]:

• The button expands overlapping the grey highlight state around it.

[Notes]:

• The button expands when hovering with Sidebar in both auto-hide and enlarged modes.
• This is reproducible with both Test Pilot and dev versions of Tab Center.
• Please see the screenshot attached:

[nishanthkarthik]

I can reproduce this too.