I recently encountered a weird behaviour in Domino Designer - again. There was one custom control I was not able to open anymore. The effect was that the XSP editor window appeared delayed but got no focus and didn't show anything. When I clicked the tab Domino Designer also almost crashed. I was able to open the XPage with the XML editor though. Just try it for yourself: create a blank XPage and paste the following code into it. Save and close the XPage and try to re-open it again as usual:
After some code reviewing my colleague found something that shouldn't be there: an event handler in a view panel. View panels do not have any events. This prevented the editor to open the XPage in the desired editor.
Fun fact: the control worked well at runtime!
After we put it where it belonged (in the panel around the view) everything was good again. Bollocks!