I'm not able to reproduce that on my end, but a quick way to tell if it's a theme issue or not is to switch to a default theme temporarily - like Twenty Fourteen or Twenty Thirteen - to see if the behaviour is the same. You may need to reset your widget and other theme-specific settings afterwards, so you might want to make a note of them.
Another troubleshooting technique is to try deactivating all your plugins. Does the problem go away? If yes, turn the plugins back on one-by-one to find the conflict.
One more idea: have you tried a different browser? If it happens only in one web browser (like Firefox or Chrome) then try deactivating all of that browsers add-ons or extensions. If the problem goes away, then you know an add-on is to blame and you can try turning them back on one by one to determine which one is at fault.
Finally, clearing your browser's cache often fixes problems like this, which are sometimes caused by incompletely loaded file in the editor.
Let me know how it goes!