It seems like clicking on any non-clickable element on the #maincontent element shifts the tab focus to the very beginning of the #maincontent element regardless of the position of clicked element (e.g., span). The culprit seems to be the tabindex setting here

which was added in order to improve the accessibility of the software.

This is not intended nor expected behavior as, for example, clicking on a widget + tab does not keep the focus in the clicked element (although invisibly so) as it would in normal website browsing.