Pop-up overlay too wide when selecting media

I have a node type with an image field that has the "Media file selector" as widget type.

Now when I create a new node and click on "Select media" and "Library", the thumbnails of all available images run over the right hand edge of the pop-up/overlay/iframe or whatever it is called... I need to select a couple of thumbnails with my mouse and sweep to the right in order to get to that edge where the scrollbar is hidden.

See the attached screenshots to understand what I'm saying. I guess the problem can be fixed by adjusting the width of the space available for browsing through the thumbnails to the width of the little pop-up screen, but I have no idea where I can do that.

Can anybody help?