Highlighting code instead drags file tab around?

A developer I work with is running into an issue where when he tries to highlight some code (with the mouse) on a line, it seems to randomly grab the file's tab and show it hovering above the code, and releasing the left-click on the mouse drops the tab to whichever side it's farthest (so it's either in the same place or to the left or to the right of another tab). It results in a highlight moving the tab around and plopping him into another file he has open.

We're not seeing what's causing it, so is there a shortcut for this that he's inadvertently activating? I've never seen it happen on my PHPStorm, and I'm not sure it's a useful feature or bug or what.