This is not first report of this issue and I think all came from people using Macs. In Cross Browser Testing, I tried using Chrome 75 with latest MacOS with various screen sizes and was not able to reproduce.

@Jpita what is the size of your browser window when you see this?

Actually, I think I know where the problem lies. We use some basic math in CSS to calculate how wide the dialog should be, in order to cover the editing surface (see T198390). On Mac, scrollbars are not taking away the width of browser viewport, as they are "floating". On other operating systems, scrollbars take some space and this difference messes up with our width calculations.