There is something fishy with the way you describe your procedure:
1) open edit person
2) click on the fact to be changed
3) click either Sources or Notes in the Details screen
4) click Edit and the little pop-up appears - but there is no Edit button after you click the Notes button because the Notes Editor opens directly. And when you click on the Sources button the Citation Manager appears without having to click on some Edit button..
5) click pop-up to open full screen - you can also drag an edge or corner of the (too small) window to resize it and that size will be remembered for the next time you open it (at least in some cases); you do not have to force it to full-screen if you don't want to.
6) click Edit and Edit is now working!!!!! - Only the Citation Manager window has an Edit button, not the Notes window. Click Edit and the source/citation currently selected in Citation Manager is opened in the Edit Source window. The Notes Editor has a menu item called "Edit" which merely collects Cut/Copy/Paste/Undo controls under it. So what was not working and now is?
The image you posted was of the Edit Source window which is what you should get if you started by clicking on the Sources button in the Edit Person screen. It merely needed to be resized.