Wondering if anyone else is having this issue. We are on version 7.9. Some of

Will it close them or will hang when opening the modeler forcing you to close it manually and opening the new instance to monitor again?

If you haven't saved the open processes it will ask if you want to leave the page. If there aren't any changes in the open processes it just closes them to open the monitored process.

To my knowledge this will only happen if you try to open models/processes from environment B while the Process Modeler from environment A is open.

I just tested it, if I have a process open from environment A and open another process model to monitor in environment A the first process model closes.

I can verify I also noticed this issue in /design when opening process models from 2 different applications (same tab/session of IE). If the models are in the same application, they will open in separate tabs - if I switch applications and attempt to open another process model, the Process Modeler will hang. The issue does not seem to be present in /designer for me.

By "different applications" you mean, different environments/urls, right?

Same environment, same tab - if I am in /design and open a process model, then navigate "back to all applications" and choose a different application in the same browser/environment/tab, and attempt to open a second process model, the process modeler window will change to a white screen with the "Appian Process Modeler" loading window and hang there forever.

In working with Andre today we determined this is session context related functionality that has been around for some time. In /designer, the process modeler will prompt/close if you are in a separate tab or if you have refreshed your current tab using F5, etc - however models opened in the same tab/session context should open in a tabular format in the modeler. In /design, when you navigate between applications session context is updated thus causing the modeler to close/hang when a new model is opened on top of a previously opened model.