Using Sketchup Web for the first time, I’ve found that clicking and dragging the mouse pointer as if to make a selection produces a kind of ‘ghosting’ effect (I’m not sure how else to describe it). It clears on releasing the mouse/trackpad button. A search of the help articles and community didn’t find anything about the issue (although I might not have used the correct terms).
It is a bug in your browser’s graphics capabilities. Try another browser, Chrome, for instance. The web app has had multiple problems with Brave browser.
Then I don’t know. It is a clear indication of a graphics driver problem so if you had a PC I would recommend updating the graphics driver. Are your MacOS components up to date?
Yes, for the current version of macOS that I’m running, everything seems to be up to date. Disabling browser graphics acceleration just results in an error.