One-tab panel

[unera]

Description

It would be nice to have a panel for only one tab.

Such panels should:

• hide list of tabs
• but show panel icons/buttons
• show pined icons/buttons

So at screenshot sidebery panel could have width about 40px instead 200px.

The feature could be useful for messengers (for example).

[mbnuqw]

Blocked by https://bugzilla.mozilla.org/show_bug.cgi?id=1408077