Long string pasted in the Search box is not completely displayed if the Sidebar is manually expanded

[SoftVision-CarmenFat]

[Affected versions]:

• Firefox 55.0.3
• Tab Center v.1.37.0

[Affected Platforms]:

• All Windows
• All Linux
• All Mac OS

[Prerequisites]:

• Tab Center (version from Bug 1380005) was already installed on a new profile.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites.
2. Drag the Sidebar border to the maximum right.
3. Copy a long string and paste it into the Search box.
4. Observe the way the string is displayed in the Search box.

[Expected result]:

• The string is correctly pasted and visible in the Search box.

[Actual result]:

• The string is visible only on the left side of the Search box, the other side that seems to be part of the extended Sidebar area doesn't display anything.

[Notes]:

• This is also reproducible by manually writing a long string in the Search box.
• Here is a screen recording of the issue: