Ladybird/Qt: TabBar doesn't extend the whole window width

[jamierocks]

See the line that runs under the tab bar.

[simonkrauter]

Introduced in https://github.com/LadybirdBrowser/ladybird/pull/629 with the line m_tabs_container->tabBar()->setMaximumWidth(window_width - 32);.

[circl-lastname]

That is kinda intentional to reserve space for the New Tab button when the tab bar is full, but it doesn't play nice with themes clearly

[jamierocks]

This still doesn't look right, or extend the full width.

[AtkinsSJ]

That's really odd. I can't believe this is so complicated. 😅