Let portal UI won't out of screen

[NeoLSN]

This is a library which used flutter portal.
fayeed/flutter_mentions#10

1. Can the flutter portal provide a way can let the portal UI always in the screen?
2. Or, let the portal UI be like a magnet in a case, just always around the main UI but won't follow the UI out of screen.

[hamishjohnson]

Despite that mentioned issue having been resolved, I still think this would be an awesome addition. Not worrying if the portal will exceed the boundaries of the screen would be so useful.

[fzyzcjy]

Hi, #44 is merged. Could you please check whether this issue is resolved :)

[fzyzcjy]

Without further information, I will temporarily close this issue to make the issue tracker clean. Feel free to reopen (or create a new issue) if you are still facing problems after the #44 refactor!