Hmm yeah, that’s bizarre. And yes, same engine (Chromium) underneath.
Interesting issue. I would look for commonalities between the two browsers, like maybe if you use the same extension in both, etc. And you could try creating a new profile and see if it happens there as well.
But, it could very well be some kind of system problem, like a bad mouse driver or something. Do you have auto-updates enabled for your OS? Is this Win7, or Win10?