messagebox and inputbox block most things in SketchUp as John said. You can move drag them around with their small title bar though. If this had been implemented as a sheet, you wouldn’t be able to drag them around. You have to dismiss a sheet before you can see all of your window again. For certain extensions sheets wouldn’t be a good idea if you at least wanted to drag it out of the way to see your model.
Running multiple instances of SketchUp would be rare. If you’re going to do it you’d have to expect some weird behaviour. Not all apps are even written to expect you to run multiple instances. A possible use would be if you had a huge model in one instance that was slowing SketchUp down and making components for it in another instance that was running fast. Maybe you have an extension rendering in SketchUp’s thread and you want another fast instance of SketchUp running to do something else. It’s a way of making use of more cores on your CPU. It can be frustrating seeing SketchUp going slow at 100% of one core while the rest of your cores sit idle.
To get multiple instances you have to type a command in Terminal like this:
open -na Sketchup.app
The command line options used are:
-n Open a new instance of the application(s) even if one is already running.
-a application
Specifies the application to use for opening the file
You could specify the full path but the above command seems to open the latest version for me.
For john_drivenupthewall a modal HtmlDialog with no defined position froze SU and he couldn’t see (or close) the dialog. It turned out it was on another desktop due to some setting he had made. I don’t think it actually was a SU bug, but just a somewhat unexpected intended behavior, but I don’t know enough about Mac to really know for sure.