At me (OSX, C4D R20, Corona 4 Beta ), the native C4D IRR freezes when you want to resize it.
Especially the bigger the scene is...
Quite enervating, when you want to render only this region in the viewport.
In Core 1.7 - there has never been this problem. Even in huge scenes.
Could also be R20... ?
Any ideas what's going on?