Some people have reported an issue in Chrome 45 that breaks the WordPress admin menu last five weeks ago. They’ll occasionally fall out-of-place if you hover the mouse cursor over menu items in the sidebar. The source of the problem stems from Slimming Paint which is enabled by default in Chrome 45. If you disable slimming paint it fixes the issue. Here’s the guide: visit chrome://flags/#disable-slimming-paint in Chrome and Enable the Disable slimming paint option, and make sure the other two Enable options are disabled because they will override the Disable option. The Chrome’s development team is aware of the issue and is working on a solution.
If the guide above is sounds confusing, please visit A Bug in Chrome 45 Causes WordPress Admin Menu to Break.