pm_section: Fix ghost tooltip on collapse/expand DM toggle.

As reported in a bug report, the tooltip on the DM collapse/expand
toggle was reappearing after switching in and out of the browser window.

This commit destroys the current instance of the tooltip when it is
hidden. This clean up ensures that the tooltip does not appear again
unless the triggered explicitly by the user.

web/src/tippyjs.ts

@@ -487,6 +487,9 @@ export function initialize(): void {
         },
         delay: EXTRA_LONG_HOVER_DELAY,
         appendTo: () => document.body,
+        onHidden(instance) {
+            instance.destroy();
+        },
     });
 
     tippy.delegate("body", {