Can't resize the form by dragging its borders

Because this is not in the answers, I'll write this here.

The problem seems to be caused by the form's AutoSizeMode being on GrowAndShrink, and not GrowOnly, which is the default setting. Resetting to GrowOnly fixed the issue.

(confirmed on MSVS2013 with .net 4.5 on Win7)

Make sure the FormBorderStyle is set to Sizable, and that the SizeGripStyle property is set to Auto or Hide.

I have the same problem if maximum size was set. Please set it to 0 or to bigger than now. After that you can move the border wherever you want. Change FormBorderStyle or SizeGripStyle can't help if maximum size is too small to new settings.