Double Scrollbar issue on editing post

Hi there,
after the update to WP 5 (and Classic Editor) I’ve noticed the following issue (I’ve discussed it with the theme developer and it is not related to the theme; I’ve also performed a deep check deactivating plugins one per one, and there is no way to resolve the issue).

The issue
The scrollbar right to the screen appears and disappears on mouse over wordpress classic tinymc editor icons (standard composer). This issue is present both when writing a post with WPBakery (see first part of the video) and when you use the classic editor, that is the wordpress composer (look at the second part of the video), becoming absolutely relevant in the second case. Practically, as shown into video, I’m not able to use link button or edit button because the button flikers/flashes. This is, I suppose, because the scrollbar on right side has an odd behavior. Please, have a look to the video I’ve recorded and let me know what you think. IMPORTANT: I use Windows 10. The issue is present on chrome, brave browser and explorer/edge. Actually it disappears on smartphone or tablet screens. So basically it is related to screen resolution (my laptop is 17’ widescreen 1680×1050). There is some malformed coding somewhere.

https://www.youtube.com/watch?v=3ESH27rHeo0

I’m not sure, but I think the error is here, however in the tinymce:

https://imageshack.com/a/img924/8134/xI2yrs.jpg

The page I need help with: [log in to see the link]