After the modal panel has opened, resize the browser window so that it becomes higher (leave the width unchanged).
You should now see that the modal panel doesn't block the newly revealed bottom part of the window...
The problem seems to be triggered by the fact that the absolutely positioned container DIV (with the class rich-modalpanel) has a size of 0x0.
An attempt at a workaround, seems to be to set "right: 0;bottom:0;" in addition to the "top:0px;left:0px" which is already being set. This makes the container DIV fill the whole window - and now the contained DIVS (with width & height 100%) will now resize nicely when the size of browser window is changed.
The only problem now, is that the container DIV, for some reason has an inline style of "background-color: inherit", which means that it most likely generates a completely opaque layer beneath the otherwise semi-transparent overlay.
To also fix this with a CSS only workaround, I ended up with