[BUG] Previous `edit` will draw cursor after switching focus to another `edit` in a different window

As mentioned in [comment](https://github.com/Immediate-Mode-UI/Nuklear/pull/779#issuecomment-3478177041) Nuklear still has a bug when switching focus between two `edits`(include `properties`) that belong to different windows. At the moment, the previous `edit` will continue to draw the caret, **but** keyboard input is also affected, the `edit` that receives input depends on which one the mouse cursor is currently hovering over.
**Note:** This only happens when switching between *different* windows, *not* when switching between `edits` inside the same window.
For more information, see the attached screenshot (tested with SDL2)

Start by reproducing the issue with the SDL2 setup described in the report, switching focus between edits in two different windows. Trace the edit focus, caret drawing, and keyboard-input paths; done means the previously focused edit no longer draws the caret or receives input after focus changes, while switching edits within one window still works.

Written by the indexing model from the issue text.