Dynamic changes to ScrollContainer content not applying

[freakboy3742]

There is currently a bug with the toga.Scroll_container() widget where if you try to change the content of it, the app displays everything weird.

To Reproduce
Steps to reproduce the behavior:

1. Create a scroll container widget and set it as the content of the main window.

#create a box and add any widgets needed
my_box=toga.Box(style=Pack(direction=COLUMN, padding=5))
my_box.add(widget)
self.scroll_container = toga.Scroll_container(vertical=True, horizontal=False, content=my_box)
self.main_window.content=self.scroll_container

Then in a handler for a button that changes the content of the scroll container:

def change_content(self, widget):
  self.scroll_container.content=another_box

Whatever is in the second box will try to display, but all of the content from the first box will still be there.

Expected behavior

Content from the second content box should be added to the scroll container; original content should be replaced.

Environment:

• Operating System: Windows
• Python version: ?
• Software versions:
  • Briefcase: ?
  • Toga: ?
  • ...

Additional context

Reported via Discord user @blindgoofball

[freakboy3742]

See #1915 for a possibly related bug.