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When I open the Mint Menu and double-click the title on the right, in my case it was "Applications," the menu thinned to look like an WinXP Start Menu. When I double-click the title again, in my case it was "Places," then it becomes a thin line along the left-side of my monitor. I right-click the desktop to open terminal and reboot to get the WinXP version back.
Yea, When I double-click one of the titles, it has been minimised to the top, left corner of the Main Menu (the little sticky boxes). When I click the little box, the Menu is restored. (the boxes don't show up until one has been minimised) I can log out of my kde, go into Gloria & get a screenshot if you want. Just let me know.
Right click the menu button, preferences>options >show sidepane
Add back the boxes you want
Repeat step 1 to get rid of the sidepane again.
Edit: Look at the screenshot in the picture above. If you click the "empty" button at the top of the sidepane it removes the sidepane. If you've put all the menu parts into the sidepane the menu collapses into the line you have.
I think this shows a limitation of the mintmenu. It would be good if you clicking that thin line brought back the sidepane like the panel toggle in Opera (it's disabled by default if you want to try it to see what I'm talking about. Tools>Appearance>Panels>Show panel toggle at edge of window).