I struggled with this when i got my mi box. I figured out it has something to do with the aspect ratio of the browser vs the aspect ratio of the tv. unmaximize chrome and make the window wider or more narrow and the weird overlap will change. find the right aspect ratio and it will go away.
tried to reinstall?
when this happened to me a while ago it turned out to be some stupid corporate policy on my laptop preventing me to cast, I wiped out win10 and reinstalled and I was able to cast w/o problems
Have you tried Brave instead of Chrome? It's a chrome's clone with adblocker included, also has Chromecast support and it's open source https://brave.com
It's a Mibox feature, happy to read there's workaround
I've always assumed this was a Mibox "feature" keen to hear if there is a workaround