There is a dotted-border selection box created between a point on the screen and the current mouse location when alt-tabbing. This only occurs when the mouse is over a portion of the screen that is used by the "ConnectionWindow".
1. Open mRemoteNG and ensure the ConnectionWindow is visible
2. Switch to a different application
3. Ensure the mouse is over the portion of the screen that will be used by ConnectionWindow
4. Alt+Tab back to mRemoteNG and move the mouse
5. You should see a dotted-border rectangle created between the mouse and the point on the ConnectionWindow where the mouse was when Alt+Tab was run
Note: at first I thought this was just when Alt-Tabbing, but this can also be recreated when hitting the Windows key when the mouse is over the ConnectionWindow
Windows 10 x64