The browser window's UI is not entirely visible if you resize to the minimum width after dragging a non-split tab in between split tabs

[Softvision-CristinaBadescu]

[Affected versions]:

• Nightly 60.0a1
• Tab Split 0.1.7

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Have a clean Firefox profile with the latest Tab Split v0.1.7 installed.
• Have 2 tabs open.

[Steps to reproduce]:

1. Open Firefox browser with the profile from prerequisites.
2. Click the "Tab Split" button.
3. Drag the non-split tab in the middle of the split tab.
4. Observe the window's UI as you resize it to its minimum width.

[Expected result]:

• All of the window's UI is constantly visible.

[Actual result]:

• As you reach towards the window's minimum width the browser window buttons ("Minimize", "Maximize", "Close"), tab strip, toolbar, vertical scrollbar and a part of the page's content are no longer visible.

[Notes]:

• On Linux OS'es all of the above elements are affected except the browser window's buttons.
• On Mac OS'es only the browser's "Menu" button and a part of page's content are no longer visible.
• Here is a screen recording with this issue: