Expensive solution for a small annoying and serious
When the sidebar was hidden, then digikam closed, after restarting clicking on a tab bar button would show the sidebar at size 0 (only handle visible), which is a difficult to understand situation.

Solution:
Use a dedicated QSplitter subclass the cooperates closely with the Sidebars it contains.

Solves the bug.
When the user now moves the splitter handle so that the sidebar collapses, this is recognized and the transformed as if the sidebar was hidden by clicking on a button.

After expanding the sidebar, no size of 0 (only handle visible) will be tolerated, in case of doubt the minimum size hint is used.

The SidebarSplitter class contains also the code to save and restore state directly from a KConfig group, removing a lot of duplicate fiveliners in various classes.