I recently updated our sites to WordPress 4.3 and started noticing the left nav in the admin was broken on hover. I thought at first it was an issue on my localhost, checked live sites and found it there too. This is what it looks like:

As you hover along the links in the left nav bar, they seem to jump around, disappear and reappear kind of on top of each other.

This is not a WordPress bug at all, but is a known bug in the current version of Google Chrome. (If you follow that link and would like to say ‘me too!’, just click on the star just left of the title ‘Issue 509179’) This should be fixed by the next major release.

In the meantime, you can fix this issue by disabling “Slimming Paint” in your Google Chrome options. To do that copy/paste this into your browser address bar (where you usually type the web address for a website):
chrome://flags/#disable-slimming-paint

Your browser will jump to the section with a highlighted title that says “Disable slimming paint”. The wording is a little confusing because you’re enabling disabling this feature. Just click the link that says ‘enable’ and then restart your browser.

Your WordPress admin menu will render normally again.