Enter doesn't work!
If you use the type hierarchy dialog in intellij it will list the classes as "Show members" feature in navigation pane normally does, which in turn results in that you cannot use the keyboard to open the file.
At least not with enter.
Do you really have to use the mouse to go there and click?
How can you open the file in Type Hierarchy using the keyboard?
The shortcut is Ctrl + Enter.
Use Esc to get the focus back to the editor.
You can also press F4 to both open the file and jump to the source code.
Those shortcuts can be adjusted at File > Settings > Appearance & Behavior > Keymap > Main menu > View > Show source (first method) or Jump to Source (second method)-