Fix position of pop-up issue editor to be always on the screen

[timbl]

"When you click on an issue, the pane/window comes up at the top of the page rather than top of screen (you sometimes have to scroll up quite a lot)

It sometimes also changes width depending on the chat function below and can take up whole screen, pop up window should be limited width so not to be whole screen"

Maybe also just limit t width of the pop-up and-or the chat in it.

[timbl]

Done a while ago. May still change the pop-up view to be one of the top level tabs in the app, for consistency.