With Domino Designer on Eclipse one change IBM made was that you needed to double click on the design elements to open them. Very irritating for a Lotus Notes developer who has been developing applications in Lotus Notes 7 or before for quite some time. But thankfully IBM provided an option in the preferences to get the previous behavior.
Open your Domino Designer on Eclipse and go to "File > Preferences". In the “General” section you see option of “Open Mode” where you can select either “Double click” or “Single click”. Select "Single click". Use the image below for reference.
And good old days are back again… :)