In Chrome, the sidebar menu items in the wordpress admin dashboard sometimes load incorrectly, causing the menu structure to appear corrupted on mouseover events. This is a known issue in Chrome v45 and above.
Here is a forum post from Samuel Wood outlining the problem and a fix (source)
"Chrome 45 has a rendering bug. There is a ticket open on Chrome's bug tracker about this issue, hopefully they will fix it soon.
In the meantime, here is a workaround to eliminate the issue in Chrome.
- Go to chrome://flags/#disable-slimming-paint
- Enable the "Disable slimming paint" option.
- Ensure that the "Enable slimming paint" option below it is not turned on.
- Relaunch Chrome.
When you're done, the final result should look like this:
This will disable the problem code in Chrome and allow the menus to render normally."
IMPORTANT - Be sure to hit the "RELAUNCH NOW" button to get the change to take effect, it is not enough to simply restart Chrome, it must be relaunched.