Narrow viewport => not showing map in browser?

I have a webapplication using the arcgis javascript API and it works great. Unfortunately I ran into a small peculiarity:

If the viewport for the map is smaller than 930 pixels it refuses to load my map. I get the zoom in/zoom out button and the scalebar, but the map area stays empty. If I resize my browser slightly and reload the page the map shows.

The annoying in all this is that, as soon as the initial load is complete, I can perfectly resize the map area to 100 pixels or less and it still works. It's just the loading step where it breaks.

I've tested this in the latest firefox, but chrome exhibits the same behavior.

Any ideas/pointers where to look?