Comment 2 for bug 388160

Have had this happen to me as well. It's quite simple to reproduce. Just start dragging as soon as you can when you click.
Unfortunately, most people get used to this and unconsciously delay the dragging a moment to avoid it. It's most visible if your window border is thin (like a one pixel border). Happens on every system I've tried, both desktop and notebook, with USB and PS2 mice.