When you have a nested vis_contents in a screen master, and one of the icons in the nested contents goes beyond the dimensions of the client screen, this can result in undefined behavior with the viewport.

1.) Once you login click "Add Master".
2.) Click "Add hooks".
3.) Move like 5 tiles to the right and you'll find yourself with a blackspace due to the big screen object.
4.) Use "RemoveHooks" and that would clear out the big screen object, but your screen remains stretched out.
5.) "RemoveHooks3" will be a verb that sets the screen master's vis_contents to null, and that will fix you.

Here is the test source that the bug was replicated:
https://www.mediafire.com/file/8hdtyojyelfo0fk/ screenmaster_test.zip/file

Viewport stretches even with a large object in the vis_contents of a screen master object.

Map/viewport is in the bottom left corner and the rest of the map is black.

The problem occurs about 95% of the time.

Another strange issue:

1.) Move 6 tiles diagonally
2.) Generate screen_master.
3.) Add hooks.

The map does not get displaced when it should.

When you do RemoveHook3, suddenly the map get's displaced.

Workarounds: Setting the screen master object's vis_contents to null in mob/Login()