[Firefox on Linux] Fixed detached window UI gets closed when its decoration is clicked (thanks richard for reporting).

src/ui/popup.js

@@ -69,7 +69,14 @@ addEventListener("unload", e => {
       } catch (e) {
         close();
       }
-      addEventListener("blur", close);
+      if (browser.windows) {
+        const myWinId = (await browser.windows.getCurrent()).id;
+        browser.windows.onFocusChanged.addListener(windowId => {
+          if (windowId !== browser.windows.WINDOW_ID_NONE && myWinId !== windowId) {
+            close();
+          }
+        });
+      }
     } else {
       tabId = tab.id;
     }