When user clicks on an inactive tab, the tab is activated and dragged right, if previously active tab was to the left from current.
This bug depends on Opera window width. For example on 1920px bug appear when 42 tabs are opened, and disappear if user reduces quantity of tabs to 41. However resizing Opera window to 500px width (minimum size) revives the bug instantly on 6 tabs opened, and reducing to 5 tabs (or less) makes tabs work well.
video with Opera bug on 6 tabs (width 500px). https://www.youtube.com/watch?v=516NTTMvHWw