@elextr
You can confirm this is GTK default behaviour with the message window notebook tabs which also do the default thing.
The behavior of the sidebar and the message window are different: 1. Sidebar: If the tab has focus, so not the content-area of the tab, then tab-browsing with the arrow-left-key or the arrow-right-key is possible. Message-window: If the tab has focus, so not the content-area of the tab, then tab-browsing with an arrow-key is not possible. 2. Sidebar: If the tab has focus, so not the content-area of the tab, I can put the content-area of the tab in focus by pressing 1x the arrow-down-key. Message-window: If the tab has focus, so not the content-area of the tab, I can not put the content-area of the tab in focus by pressing 1x an arrow-key.
The common behavior of sidebar and message window is: When clicking on a tab-title, it can be – under certain conditions –, that the content-area of the tab is in focus.