After a recent Ubuntu update I didn't see the main menu bar of the RubyMine IDE (File | Edit | View | ...) anymore.

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This could be solved by changing a RubyMine registry entry:

• Search "registry" within the "Actions" search
  • press ctrl + alt + n > click on Actions > type registry > click on Registry...
• Scroll down to linux.native.menu and disable the checkbox

After rebooting RubyMine, you'll have gotten the menu bar back.