Problem description: Decompile works fine when viewing a class (i.e. Ctrl+Shift+T), but not when stepping into code from the debugging perspective - instead the "Class File Viewer" is opened. Version Used: Eclipse Oxygen and Enhanced Class Decompiler 3.0.0
This will work on oxygen and later version as well:
Click on "Window > Preferences > General > Editors > File Associations"
Change default to your for both .class association.
"*.class" : "Class Decompiler Viewer" is selected by default.
"*.class without source" : "Class Decompiler Viewer" is selected by default.