do not allow popups to resize over base windows that it was not already covering

[carbon-steel]

This change should have 2 parts:

1. The title
2. New popups should be free to cover any uncovered base windows.

This removes 2 unintuitive behaviors from the plugin:

1. A popup is created but not over the window that user created it from even though doing so would make the popup larger
2. A window that was not covered and so was being used by the user got covered after a resize
3. A popup could not be created over an uncovered base window. This can happen if that window is "reserved" by another popup.

Overall, these changes should just make the behavior of the plugin match what the user would expect based on what they see.