diff --git a/chrome/content/zotero/xpcom/notifier.js b/chrome/content/zotero/xpcom/notifier.js
index 2d7af1969d..44f48b24f1 100644
--- a/chrome/content/zotero/xpcom/notifier.js
+++ b/chrome/content/zotero/xpcom/notifier.js
@@ -110,6 +110,15 @@ Zotero.Notifier = new function () {
 	};
 	
 	
+	/**
+	 * @return {String[]} - Ids of registered observers that belong to a closed window
+	 */
+	this.getLeakedObserverIDs = function () {
+		return Object.keys(_observers)
+			.filter(id => Zotero.Utilities.Internal.isObjectLeakingWindow(_observers[id].ref));
+	};
+	
+	
 	/**
 	* Trigger a notification to the appropriate observers
 	*
diff --git a/chrome/content/zotero/xpcom/prefs.js b/chrome/content/zotero/xpcom/prefs.js
index 2629eec421..caf1fbf58e 100644
--- a/chrome/content/zotero/xpcom/prefs.js
+++ b/chrome/content/zotero/xpcom/prefs.js
@@ -512,6 +512,23 @@ Zotero.Prefs = new function () {
 	}
 	
 	
+	/**
+	 * @return {String[]} - Name of the observed pref for each registered observer that belongs to
+	 *     a closed window, with one entry per observer
+	 */
+	this.getLeakedObserverNames = function () {
+		var names = [];
+		for (let [name, handlers] of Object.entries(_observers)) {
+			for (let handler of handlers) {
+				if (Zotero.Utilities.Internal.isObjectLeakingWindow(handler)) {
+					names.push(name);
+				}
+			}
+		}
+		return names;
+	};
+	
+	
 	this.getVirtualCollectionState = function (type) {
 		const prefKeys = {
 			duplicates: 'duplicateLibraries',
diff --git a/chrome/content/zotero/xpcom/utilities_internal.js b/chrome/content/zotero/xpcom/utilities_internal.js
index 5eda717f7f..c259bcc6b2 100644
--- a/chrome/content/zotero/xpcom/utilities_internal.js
+++ b/chrome/content/zotero/xpcom/utilities_internal.js
@@ -2649,18 +2649,21 @@ Zotero.Utilities.Internal = {
 	},
 
 	/**
-	 * Check whether an object belongs to a closed window, and is therefore
-	 * keeping it alive.
+	 * Check whether an object or function belongs to a closed window, and is
+	 * therefore keeping it alive.
+	 *
+	 * A window only counts as leaked once it's closed and detached from its
+	 * docShell, which happens after its 'unload' handlers have run.
 	 *
 	 * @param {any} obj
 	 * @returns {boolean}
 	 */
 	isObjectLeakingWindow(obj) {
-		if (typeof obj !== 'object' || obj === null) {
+		if (obj === null || (typeof obj !== 'object' && typeof obj !== 'function')) {
 			return false;
 		}
 		let global = Cu.getGlobalForObject(obj);
-		return global.constructor.name === 'Window' && global.closed;
+		return global.constructor.name === 'Window' && global.closed && !global.docShell;
 	}
 };
 
diff --git a/chrome/content/zotero/zoteroPane.js b/chrome/content/zotero/zoteroPane.js
index 0d7ab0b0ef..bf86cf5f4c 100644
--- a/chrome/content/zotero/zoteroPane.js
+++ b/chrome/content/zotero/zoteroPane.js
@@ -791,6 +791,9 @@ var ZoteroPane = new function () {
 		if (_syncRemindersObserverID) {
 			Zotero.Notifier.unregisterObserver(_syncRemindersObserverID);
 		}
+		if (_apiKeyObserverID) {
+			Zotero.Notifier.unregisterObserver(_apiKeyObserverID);
+		}
 		
 		this.uninitContainers();
 		
@@ -3279,9 +3282,10 @@ var ZoteroPane = new function () {
 
 
 	var _syncRemindersObserverID = null;
+	var _apiKeyObserverID = null;
 	this.initSyncReminders = function (startup) {
-		if (startup) {
-			Zotero.Notifier.registerObserver(
+		if (startup && !_apiKeyObserverID) {
+			_apiKeyObserverID = Zotero.Notifier.registerObserver(
 				{
 					notify: (event) => {
 						// When the API Key is deleted we need to add an observer
diff --git a/test/tests/tabsTest.js b/test/tests/tabsTest.js
index b7ef45bb29..3d91882c0a 100644
--- a/test/tests/tabsTest.js
+++ b/test/tests/tabsTest.js
@@ -40,4 +40,50 @@ describe("Zotero_Tabs", function() {
 			assert.notOk(tab.querySelector('img'));
 		});
 	});
+
+	describe("Window teardown", function () {
+		it("should not leave observers registered after a window is closed", async function () {
+			this.timeout(60000);
+			let collection = await createDataObject('collection');
+			let item = await createDataObject('item', { collections: [collection.id] });
+			let notifierBefore = new Set(Zotero.Notifier.getLeakedObserverIDs());
+			let prefsBefore = Zotero.Prefs.getLeakedObserverNames();
+			
+			// Render the library item pane in a second window
+			let win2 = await loadZoteroPane();
+			await selectCollection(win2, collection);
+			await win2.ZoteroPane.selectItem(item.id);
+			
+			// The window's observers are still registered while it tears down, and shouldn't be
+			// reported as leaked before it's actually gone
+			let leakedWhileUnloading = null;
+			win2.addEventListener('pagehide', () => {
+				leakedWhileUnloading = Zotero.Notifier.getLeakedObserverIDs()
+					.filter(id => !notifierBefore.has(id));
+			});
+			// An object that nothing unregisters, so that it starts reporting as leaked as soon
+			// as the window has finished tearing down
+			let root = win2.document.documentElement;
+			win2.close();
+			await waitForCallback(
+				() => Zotero.Utilities.Internal.isObjectLeakingWindow(root), 50, 20
+			);
+			
+			assert.deepEqual(
+				leakedWhileUnloading, [], 'no observers are reported as leaked while unloading'
+			);
+			let notifierAfter = Zotero.Notifier.getLeakedObserverIDs()
+				.filter(id => !notifierBefore.has(id));
+			assert.isEmpty(notifierAfter, 'no notifier observers belong to a closed window');
+			
+			let prefsAfter = Zotero.Prefs.getLeakedObserverNames();
+			for (let name of prefsBefore) {
+				let index = prefsAfter.indexOf(name);
+				if (index != -1) {
+					prefsAfter.splice(index, 1);
+				}
+			}
+			assert.isEmpty(prefsAfter, 'no pref observers belong to a closed window');
+		});
+	});
 });