If a scrolling action is occurring (via holding down left mouse button) and the mouse cursor moves outside of the host client window, the session locks up. It becomes unresponsive (as though the entire client window has lost focus) until the window receives a mouse click to bring back "focus", at which point normal interaction can then resume. This can be quite annoying especially since the scroll bar in a full screen window will always be right at the very edge of the client window making it difficult to not move the cursor out of the client window.
In addition, when the cursor leaves the client window while scrolling a "generic" icon appears in the task bar coinciding with the unresponsiveness, and this icon disappears when the window receives a mouse click that restores normal behavior.
This occurs in v6.6.18120.6697 (as well as all previous versions going back at least several years). I'm happy to provide any additional information requested.
Customer support service by UserEcho