Subtle regression in [0] caused context override to fail when
passing in the current context when a temporary screen was used.
While passing in a temporary screen as an override isn't supported,
this shouldn't raise an exception if the argument matches the current
context.
Also added sanity check that the window passed in exists
and code comments to give an explanation of the current behavior.
[0]: 6af92e1360
328d5bcc3c