But: in case the window is already showing but hidden under another window, or showing but on a different workspace / virtual desktop, clicking "Show window" indeed does nothing. If it would close itself and show itself again in that case, it would pop up on the current desktop. Which is probably what the user wanted.
The same description of behavior applies to the button "Display the main window" in the bottom right of the "Project → Lookup project …" window.
Works for me (tom-ui 0.5.0 on Ubuntu 18.10).
But: in case the window is already showing but hidden under another window, or showing but on a different workspace / virtual desktop, clicking "Show window" indeed does nothing. If it would close itself and show itself again in that case, it would pop up on the current desktop. Which is probably what the user wanted.
The same description of behavior applies to the button "Display the main window" in the bottom right of the "Project → Lookup project …" window.