Bug: tab-preview-panel fires popupshown twice but popuphidden once

When hovering Tab A then quickly moving to Tab B while Tab A's
preview panel is still in the "showing" state, moveToAnchor()
re-triggers the popup lifecycle. The popupshowing listener from
Tab A's activation is still registered, causing a duplicate
popupshown event. Only one popuphidden fires when the panel
finally closes.

This causes Firefox's own sidebar _hoverBlockerCount to get stuck
(2 increments, 1 decrement), permanently preventing sidebar
autohide from working.

Fix: remove the stale popupshowing listener when re-activating
on a different tab while the panel is in "showing" or "open" state.

Author: tompassarelli

browser/components/tabbrowser/content/tab-hover-preview.mjs

@@ -393,6 +393,10 @@ class TabPanel extends HoverPanel {
       this.panelElement.state == "open" ||
       this.panelElement.state == "showing"
     ) {
+      // Remove stale listener from previous activation to prevent
+      // duplicate popupshown events when moveToAnchor re-triggers
+      // the popup lifecycle during the "showing" state.
+      this.panelElement.removeEventListener("popupshowing", this);
       this.#updatePreview();
     } else {
       this.panelSet.panelOpener.execute(() => {