Pinned tab thumbnail exceeds its designed space and overlapps the "New Tab" button in specific conditions

[SoftVision-PaulOiegas]

[Notes]:

• The titlebar of the browser window could be the one that is causing this issue, its thickness to be more specific. It's reproducing only while you are in windowed mode, because the titlebar is thicker. And it does not reproduce while you are in maximized window, when the titlebar is thinner.
• Same thing happens while the window is in half the screen mode (WIndows).

[Affected versions]:

• Firefox v52.0.3 and above
• Tab Center v1.31.0 (release version)
• Tab Center v1.33.0 RC build

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Tab Center was previously installed from Test Pilot website or from custom repo build ("xpinstall.signatures.required" is set to "false").
• The "Show windows and tabs from last time" option is set from Firefox > Options > General tab.

[Steps to reproduce]:

1. Open the browser in windowed mode (not maximized) with the profile from prerequisites.
2. Open at least 12 tabs (can be empty) and pin one of them.
3. Restart or close and reopen the browser.
4. Observe the area above the pinned tab thumbnail.

[Expected result]:
The area above the pinned tab thumbnail correctly displays the "New Tab" button.

[Actual result]:
The pinned tab thumbnail wrongly exceeds designed space and overlaps the "New Tab" button.

[Notes]:

• This issue occurs in windowed mode only if the sidebar contains between 12 and 14 tabs and in specific combinations (see screen recording https://goo.gl/udjB46).
  -- 12 to 14 tabs with 1 pinned
  -- 12 to 14 tabs with 2 pinned
  -- 13 tabs with 3 pinned
  -- 14 tabs with 4 pinned
• The issue occurs in half screen display even with 16 tabs open because of the vertical extra space (see screen recording https://goo.gl/wAK0sI).
• The thumbnail image of the last pinned tab (in case of 2 to 4) is the one that overlaps the "New Tab" button.

[SoftVision-PaulOiegas]

Did some further investigations and my brain froze after I managed to contradict my original belief. 😄 It looks more and more now like a major issue that could be easily encountered after all.

If you end up in this state once, when the thumbnail overflows sidebar elements, then even if you open unlimited tabs, this still occurs if you filter the tabs. And the most interesting thing is that even if you enter a string that is not matching any tabs name, the pinned tabs are displayed as results...

You can see the above scenario in this recording https://goo.gl/Z54EBe.