@techee @b4n @elextr
I think I figured this out, but can't find a way to fix it. All XML files downloaded from the internet will open in a new instance. For exempel, I use this online converter to convert svg to Android xml drawable: http://inloop.github.io/svg2android/
Every xml I converted and downloaded from there will open in a new instance when double clicked in finder, or dragged to the Geany icon in the dock. Same issue with xml's downloaded elsewhere from the internet. A locally created and saved xml will open in the main instance like it should.
I understand this is a Mac OS side thing, but strange thing is that if I set the default application to Textwrangler, or any other text editor, they open in the same instance.