loukiehmg

To fix the problem just go under Download Manager or Download settings -> Misc Settings ->Disable Style & Script at the bottom of the settings page and tick Bootstrap JS, Bootstrap CSS and Font Awesome to disable it and you will still be able to use the plugin without breaking your menu

Can confirm its a plugin, in may case it was Download Manager By Shaon

I have notice it is doing the same if you minimize your browser to mobile size(responsive) so its not just a mobile browser thing.

Have the same problem not sure if it is plugin related but i have disabled some plugins and it is still broken.