Your system information

• Steam client version:
  dec 9 2016
• Distribution (e.g. Ubuntu):
  ubuntu 16.04
• Opted into Steam client beta?: [Yes/No]
  no
• Have you checked for system updates?: [Yes/No]
  no

Please describe your issue in as much detail as possible:

Describe what you expected should happen and what did happen. Please link any large code pastes as a Github Gist

Steps for reproducing this issue:

1. open steam, any window main library or chat window for example
2. cursor over any of the window corners (also happens on edges)
3. dont move the mouse, just click and hold. The window will randomly resize.

Also frustratingly note that steam doesnt seem to remember the window size, i always find myself making the window small as possible on each launch....