I’ve been struggling to find out why my menu is unstable in all the sites I’ve been managing since the 4.3 update in WordPress.

Luckily, WPTavern figured it out!

From their post:

The source of the problem stems from Slimming Paint which is enabled by default in Chrome 45. Disabling slimming paint fixes the issue.

Read the full WPTavern article here

Thank you, WPTavern.  That was driving me crazy!