Logisim sidebar disappeared
This is a bug in Logisim. It will allow you to drag the split between the main window and sidebar and release it while it's off the edge of the screen, making it impossible to pick up and drag again.

On macOS, you can fix this by closing Logisim and running the following command in the terminal:

defaults write com.cburch.logisim /com/cburch/logisim/ -dict-add windowMainSplit 0.5


This will reset the split to the middle of the window, and you can adjust it to your preferred location from there.

I haven't tested on Windows systems, but I believe you can edit Java application preferences there using RegEdit, somewhere under the registry key:

HKEY_CURRENT_USER\Software\JavaSoft\Prefs