Enable manual sorting in task manager.
When trying to click on a task in task manager it often happens when the system is under load, that a simple left click isn't registered as a click, but it initiates a drag and drop action instead.
Although it was a simple click and therefore the mouse button was released again the drag icon is then still locked to the cursor and only another click releases it again on whichever surface the cursor is on currently.
I recall i was able to reproduce this bug sometime ago (about a half year ago) on my previous laptop (Dual Core Celeron 2.1GHz, 4Gb RAM) - i can't reproduce it now on i7 CPU with stress load utility (stress --cpu 4)
also perhaps there is HDD vs SDD difference
OS: Netrunner Rolling
*** This bug has been marked as a duplicate of bug 402376 ***