diff --git a/chrome/content/zotero/containers/tagSelectorContainer.jsx b/chrome/content/zotero/containers/tagSelectorContainer.jsx
index 467ca59263..a7e529ac5c 100644
--- a/chrome/content/zotero/containers/tagSelectorContainer.jsx
+++ b/chrome/content/zotero/containers/tagSelectorContainer.jsx
@@ -642,6 +642,46 @@ Zotero.TagSelector = class TagSelectorContainer extends React.PureComponent {
 		await Zotero.Tags.setColor(this.libraryID, io.name, io.color, io.position);
 	}
 
+	async openTagSplitterWindow() {
+		const oldTagName = this.contextTag.name; // contextTag contains { name, width, color }
+		const dataIn = {
+			oldTag: this.contextTag.name,
+			isLongTag: false
+		};
+		const dataOut = { result: null };
+		
+		window.openDialog(
+			'chrome://zotero/content/longTagFixer.xhtml',
+			'',
+			'chrome,modal,centerscreen',
+			dataIn, dataOut
+		);
+
+		if (!dataOut.result) {
+			return;
+		}
+
+		const oldTagID = Zotero.Tags.getID(oldTagName);
+
+		if (dataOut.result.op === 'split') {
+			const itemIDs = await Zotero.Tags.getTagItems(this.libraryID, oldTagID);
+			await Zotero.DB.executeTransaction(async () => {
+				for (const itemID of itemIDs) {
+					const item = await Zotero.Items.getAsync(itemID);
+					const tagType = item.getTagType(oldTagName);
+					for (const newTagName of dataOut.result.tags) {
+						item.addTag(newTagName, tagType);
+					}
+					item.removeTag(oldTagName);
+					await item.save();
+				}
+				await Zotero.Tags.purge(oldTagID);
+			});
+		} else {
+			throw new Error('Unsupported op: ' + dataOut.result.op);
+		}
+	}
+
 	async openRenamePrompt() {
 		var promptService = Cc['@mozilla.org/embedcomp/prompt-service;1']
 			.getService(Ci.nsIPromptService);
diff --git a/chrome/content/zotero/longTagFixer.js b/chrome/content/zotero/longTagFixer.js
index 9acb1fd9b3..d2dd331c83 100644
--- a/chrome/content/zotero/longTagFixer.js
+++ b/chrome/content/zotero/longTagFixer.js
@@ -1,191 +1,212 @@
 /*
     ***** BEGIN LICENSE BLOCK *****
-    
-    Copyright © 2009 Center for History and New Media
-                     George Mason University, Fairfax, Virginia, USA
-                     http://zotero.org
-    
+
+    Copyright © 2022 Corporation for Digital Scholarship
+                     Vienna, Virginia, USA
+                     https://www.zotero.org
+
     This file is part of Zotero.
-    
+
     Zotero is free software: you can redistribute it and/or modify
     it under the terms of the GNU Affero General Public License as published by
     the Free Software Foundation, either version 3 of the License, or
     (at your option) any later version.
-    
+
     Zotero is distributed in the hope that it will be useful,
     but WITHOUT ANY WARRANTY; without even the implied warranty of
     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
     GNU Affero General Public License for more details.
-    
+
     You should have received a copy of the GNU Affero General Public License
     along with Zotero.  If not, see <http://www.gnu.org/licenses/>.
-    
+
     ***** END LICENSE BLOCK *****
 */
 
+const HTML_NS = 'http://www.w3.org/1999/xhtml';
 
-var Zotero_Long_Tag_Fixer = new function () {
-	var _oldTag = window.arguments[0];
-	var _dataOut = window.arguments[1];
+var Zotero_Long_Tag_Fixer = new function () { // eslint-disable-line camelcase, no-unused-vars
+	const { oldTag, isLongTag } = window.arguments?.[0] ?? { isLongTag: true, oldTag: '' };
+	const dataOut = window.arguments?.[1] || {};
 	
 	this.init = function () {
-		document.getElementById('zotero-old-tag').value = _oldTag;
-		document.getElementById('zotero-old-tag-delimiter').nextSibling.value = Zotero.getString('general.character.singular');
-		
-		var delimiter = Zotero.Prefs.get('lastLongTagDelimiter');
-		document.getElementById('zotero-old-tag-delimiter').value = delimiter;
-		
-		var lastMode = Zotero.Prefs.get('lastLongTagMode');
-		if (!lastMode) {
-			lastMode = 0;
-		}
-		this.switchMode(lastMode);
-	}
+		const lastMode = Zotero.Prefs.get('lastLongTagMode') || 0;
+		const delimiter = Zotero.Prefs.get('lastLongTagDelimiter');
+
+		this.dialog = document.getElementById('zotero-long-tag-fixer');
+		this.intro = document.getElementById('intro');
+		this.tabs = document.getElementById('zotero-new-tag-actions');
+		this.oldTagInput = document.getElementById('zotero-old-tag');
+		this.oldTag = document.getElementById('zotero-old-tag');
+		this.delimiterLabel = document.getElementById('delimiter-label');
+		this.oldTagDelimiter = document.getElementById('zotero-old-tag-delimiter');
+		this.listbox = document.getElementById('zotero-new-tag-list');
+		this.newTagInput = document.getElementById('zotero-new-tag-editor');
+		this.newTagCharacterCount = document.getElementById('zotero-new-tag-character-count');
+		this.zoteroNewTagInfo = document.getElementById('zotero-new-tag-characters');
+
+		document.addEventListener('dialogaccept', () => this.accept());
+		document.addEventListener('dialogcancel', () => this.cancel());
+		this.tabs.addEventListener('select', (ev) => {
+			if (ev.target === this.tabs.querySelector('tabpanels')) {
+				this.switchMode(ev.currentTarget.selectedIndex);
+			}
+		});
+
+		this.dialog.classList.toggle('is-long-tag', isLongTag);
+
+		this.oldTagDelimiter.addEventListener('input', () => this.onUpdateDelimiter());
+		this.newTagInput.addEventListener('input', ev => this.updateEditLength(ev.currentTarget.value.length));
+
+		this.oldTagInput.value = oldTag;
+		this.oldTagDelimiter.value = delimiter;
+
+		this.updateLabel();
+		this.switchMode(isLongTag ? lastMode : 0);
+	};
 	
 	this.switchMode = function (index) {
-		var dialog = document.getElementById('zotero-long-tag-fixer');
-		
-		document.getElementById('zotero-new-tag-actions').selectedIndex = index;
+		this.tabs.selectedIndex = index;
+		let buttonLabel = "";
 		
 		switch (index) {
+			default:
 			case 0:
-				var buttonLabel = "saveTags";
+				buttonLabel = 'saveTags';
 				this.updateTagList();
-				document.getElementById('zotero-old-tag-delimiter').select();
+				this.oldTagDelimiter.select();
 				break;
 				
 			case 1:
-				var buttonLabel = "saveTag";
-				document.getElementById('zotero-new-tag-editor').value = _oldTag;
-				this.updateEditLength(_oldTag.length)
+				buttonLabel = 'saveTag';
+				this.newTagInput.value = oldTag;
+				this.updateEditLength(oldTag.length);
 				break;
 				
 			case 2:
-				var buttonLabel = "deleteTag";
-				dialog.getButton('accept').disabled = false;
+				buttonLabel = 'deleteTag';
+				this.dialog.getButton('accept').disabled = false;
 				break;
 		}
 		
-		document.getElementById('zotero-long-tag-fixer').getButton('accept').label = Zotero.getString('sync.longTagFixer.' + buttonLabel);
+		this.dialog.getButton('accept').label = Zotero.getString('sync.longTagFixer.' + buttonLabel);
 		window.sizeToContent();
-		Zotero.Prefs.set('lastLongTagMode', index);
-	}
-	
+		if (isLongTag) {
+			Zotero.Prefs.set('lastLongTagMode', index);
+		}
+	};
 	
 	/**
 	 * Split tags and populate list
 	 */
 	this.updateTagList = function () {
-		var listbox = document.getElementById('zotero-new-tag-list');
-		while (listbox.childNodes.length) {
-			listbox.removeChild(listbox.lastChild);
-		}
+		let tags = [];
 		
-		var delimiter = document.getElementById('zotero-old-tag-delimiter').value;
+		const delimiter = document.getElementById('zotero-old-tag-delimiter').value;
 		if (delimiter) {
 			Zotero.Prefs.set('lastLongTagDelimiter', delimiter);
-			var re = new RegExp("\\s*" + delimiter.replace(/([\.\-\[\]\(\)\?\*\+])/g, "\\$1") + "\\s*");
-			var tags = _oldTag.split(re);
+			const re = new RegExp("\\s*" + delimiter.replace(/([\.\-\[\]\(\)\?\*\+])/g, "\\$1") + "\\s*");
+			tags = [...new Set(oldTag.split(re).filter(t => t.length > 0))];
 		}
 		
-		var acceptButton = document.getElementById('zotero-long-tag-fixer').getButton('accept');
+		const acceptButton = document.getElementById('zotero-long-tag-fixer').getButton('accept');
 		if (!delimiter || tags.length < 2) {
 			acceptButton.disabled = true;
-			return;
+			// return;
 		}
 		else {
 			acceptButton.disabled = false;
 		}
 		
 		tags.sort();
-		for (var i=0; i<tags.length; i++) {
-			if (i != 0 && tags[i] == tags[i-1]) {
-				continue;
-			}
-			if (!tags[i]) {
-				continue;
-			}
-			var li = listbox.appendItem(tags[i]);
-			li.setAttribute('type', 'checkbox');
-			li.setAttribute('checked', 'true');
+		
+		while (this.listbox.childNodes.length) {
+			this.listbox.removeChild(this.listbox.lastChild);
 		}
+
+		tags.forEach((tag) => {
+			const li = document.createElement('richlistitem');
+			const div = document.createElement('div');
+			const checkbox = document.createElement('input');
+			checkbox.type = 'checkbox';
+			checkbox.checked = true;
+			checkbox.id = 'tag-' + tag;
+			const label = document.createElement('label');
+			label.setAttribute('for', 'tag-' + tag);
+			label.textContent = tag;
+			// Don't toggle checkbox for single-click on label
+			
+			div.appendChild(checkbox);
+			div.appendChild(label);
+			li.appendChild(div);
+			this.listbox.append(li);
+		});
 		
 		window.sizeToContent();
-	}
+	};
+
+	this.updateLabel = function () {
+		this.delimiterLabel.innerHTML = this.oldTagDelimiter.value.length > 1
+			? Zotero.getString('general.character.plural')
+			: Zotero.getString('general.character.singular');
+	};
+
+	this.onUpdateDelimiter = function () {
+		this.updateLabel();
+		this.updateTagList();
+	};
 	
 	
 	this.deselectAll = function () {
-		var lis = document.getElementById('zotero-new-tag-list').getElementsByTagName('listitem');
-		for (var i=0; i<lis.length; i++) {
-			lis[i].checked = false;
-		}
-	}
+		this.listbox.querySelectorAll('[type=checkbox]').forEach(checkbox => checkbox.checked = false);
+	};
 	
 	
 	this.selectAll = function () {
-		var lis = document.getElementById('zotero-new-tag-list').getElementsByTagName('listitem');
-		for (var i=0; i<lis.length; i++) {
-			lis[i].checked = true;
-		}
-	}
+		this.listbox.querySelectorAll('[type=checkbox]').forEach(checkbox => checkbox.checked = true);
+	};
 	
 	
 	this.updateEditLength = function (len) {
-		document.getElementById('zotero-new-tag-character-count').value = len;
-		var invalid = len == 0 || len > Zotero.Tags.MAX_SYNC_LENGTH;
-		document.getElementById('zotero-new-tag-characters').setAttribute('invalid', invalid);
-		document.getElementById('zotero-long-tag-fixer').getButton('accept').disabled = invalid;
-	}
+		this.newTagCharacterCount.innerText = len;
+		const invalid = len == 0 || len > Zotero.Tags.MAX_SYNC_LENGTH;
+		this.zoteroNewTagInfo.classList.toggle('invalid', invalid);
+		this.dialog.getButton('accept').disabled = invalid;
+	};
 	
 	
 	this.cancel = function () {
-		_dataOut.result = false;
-	}
+		dataOut.result = false;
+	};
 	
 	
-	this.save = function () {
+	this.accept = function () {
 		try {
-		
-		var result = {};
-		
-		var index = document.getElementById('zotero-new-tag-actions').selectedIndex;
-		
-		switch (index) {
-			// Split
-			case 0:
-				// Get checked tags
-				var listbox = document.getElementById('zotero-new-tag-list');
-				var len = listbox.childElementCount;
-				var newTags = [];
-				for (var i=0; i<len; i++) {
-					var li = listbox.childNodes[i];
-					if (li.getAttribute('checked') == 'true') {
-						newTags.push(li.getAttribute('label'));
-					}
-				}
+			const result = {};
+			switch (this.tabs.selectedIndex) {
+				// Split
+				case 0:
+					result.op = 'split';
+					result.tags = Array.from(this.listbox.querySelectorAll('[type=checkbox]'))
+						.filter(c => c.checked)
+						.map(n => n.nextSibling.textContent);
+					break;
+				// Edit
+				case 1:
+					result.op = 'edit';
+					result.tag = this.newTagInput.value;
+					break;
 				
-				result.op = 'split';
-				result.tags = newTags;
-				break;
-			
-			// Edit
-			case 1:
-				result.op = 'edit';
-				result.tag = document.getElementById('zotero-new-tag-editor').value;
-				break;
-			
-			// Delete
-			case 2:
-				result.op = 'delete';
-				break;
-		}
-		
-		_dataOut.result = result;
-		
+				// Delete
+				case 2:
+					result.op = 'delete';
+					break;
+			}
+			dataOut.result = result;
 		}
 		catch (e) {
 			Zotero.debug(e);
 			throw (e);
 		}
-	}
-}
+	};
+};
diff --git a/chrome/content/zotero/longTagFixer.xhtml b/chrome/content/zotero/longTagFixer.xhtml
new file mode 100644
index 0000000000..ca40793638
--- /dev/null
+++ b/chrome/content/zotero/longTagFixer.xhtml
@@ -0,0 +1,65 @@
+<?xml version="1.0"?>
+<?xml-stylesheet href="chrome://global/skin/" type="text/css"?>
+<?xml-stylesheet href="chrome://zotero-platform/content/zotero-react-client.css"?>
+
+<!DOCTYPE window SYSTEM "chrome://zotero/locale/zotero.dtd">
+
+<window
+	xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+	xmlns:html="http://www.w3.org/1999/xhtml"
+	title="&zotero.sync.error;"
+	onload="Zotero_Long_Tag_Fixer.init()">
+<dialog
+	id="zotero-long-tag-fixer"
+	buttons="cancel,accept"
+>	
+	<html:div id="intro">
+		<label value="&zotero.sync.longTagFixer.followingTagTooLong;"/>
+		<html:textarea id="zotero-old-tag" readonly="readonly" />
+		<label>&zotero.sync.longTagFixer.syncedTagSizeLimit;</label>
+		<label value="&zotero.sync.longTagFixer.splitEditDelete;"/>		
+	</html:div>
+	<tabbox id="zotero-new-tag-actions">
+		<tabs>
+			<tab label="&zotero.sync.longTagFixer.split;"/>
+			<tab label="&zotero.general.edit;"/>
+			<tab label="&zotero.general.delete;"/>
+		</tabs>
+		<tabpanels>
+			<!-- Split -->
+			<vbox class="split-tab">
+				<hbox class="delimiter-input-wrap">
+					<label>&zotero.sync.longTagFixer.splitAtThe;</label>
+					<html:input
+						type="text"
+						class="delimiter-input"
+						id="zotero-old-tag-delimiter" />
+					<label id="delimiter-label"></label>
+				</hbox>
+				<richlistbox id="zotero-new-tag-list" class="tag-list" flex="1" tabindex="0" />
+				<label value="&zotero.sync.longTagFixer.uncheckedTagsNotSaved;"/>
+				<hbox>
+					<button label="&zotero.general.deselectAll;" oncommand="Zotero_Long_Tag_Fixer.deselectAll()"/>
+					<button label="&zotero.general.selectAll;" oncommand="Zotero_Long_Tag_Fixer.selectAll()"/>
+				</hbox>
+			</vbox>
+			
+			<!-- Edit -->
+			<vbox class="edit-tab">
+				<html:textarea id="zotero-new-tag-editor"/>
+				<html:div id="zotero-new-tag-characters">
+					<html:span id="zotero-new-tag-character-count" />
+					<html:span>&zotero.sync.longTagFixer.characters;</html:span>
+				</html:div>
+			</vbox>
+			
+			<!-- Delete -->
+			<vbox align="center" pack="center">
+				<label value="&zotero.sync.longTagFixer.tagWillBeDeleted;"/>
+			</vbox>
+		</tabpanels>
+	</tabbox>
+	<script src="include.js"/>
+	<script src="longTagFixer.js"/>
+</dialog>
+</window>
\ No newline at end of file
diff --git a/chrome/content/zotero/longTagFixer.xul b/chrome/content/zotero/longTagFixer.xul
deleted file mode 100644
index 2954190c4b..0000000000
--- a/chrome/content/zotero/longTagFixer.xul
+++ /dev/null
@@ -1,76 +0,0 @@
-<?xml version="1.0"?>
-<?xml-stylesheet href="chrome://global/skin/" type="text/css"?>
-<?xml-stylesheet href="chrome://zotero/skin/longTagFixer.css" type="text/css"?>
-<?xml-stylesheet href="chrome://zotero-platform-version/content/style.css"?>
-
-<!DOCTYPE window SYSTEM "chrome://zotero/locale/zotero.dtd">
-
-<dialog id="zotero-long-tag-fixer" xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
-		buttons="accept,cancel"
-		onload="Zotero_Long_Tag_Fixer.init()"
-		ondialogcancel="Zotero_Long_Tag_Fixer.cancel()"
-		ondialogaccept="Zotero_Long_Tag_Fixer.save()"
-		title="&zotero.sync.error;">
-	
-	<script src="include.js"/>
-	<script src="longTagFixer.js"/>
-	
-	<label value="&zotero.sync.longTagFixer.followingTagTooLong;"/>
-	<groupbox>
-		<textbox id="zotero-old-tag" multiline="true" rows="4" readonly="true" class="plain"/>
-	</groupbox>
-	<label>&zotero.sync.longTagFixer.syncedTagSizeLimit;</label>
-	
-	<separator class="thin"/>
-	
-	<label value="&zotero.sync.longTagFixer.splitEditDelete;"/>
-	
-	<separator/>
-	
-	<tabbox id="zotero-new-tag-actions">
-		<tabs oncommand="Zotero_Long_Tag_Fixer.switchMode(this.selectedIndex)">
-			<tab label="&zotero.sync.longTagFixer.split;"/>
-			<tab label="&zotero.general.edit;"/>
-			<tab label="&zotero.general.delete;"/>
-		</tabs>
-		<tabpanels>
-			<!-- Split -->
-			<vbox>
-				<hbox align="center">
-					<label>&zotero.sync.longTagFixer.splitAtThe;</label>
-					<textbox id="zotero-old-tag-delimiter" size="1"
-						oninput="this.nextSibling.value = Zotero.getString('general.character.' + (this.value.length > 1  ? 'plural' : 'singular')); Zotero_Long_Tag_Fixer.updateTagList();"/>
-					<label/>
-				</hbox>
-				
-				<separator class="thin"/>
-				
-				<listbox id="zotero-new-tag-list" rows="8"/>
-				
-				<separator class="thin"/>
-				
-				<label value="&zotero.sync.longTagFixer.uncheckedTagsNotSaved;"/>
-				
-				<hbox>
-					<button label="&zotero.general.deselectAll;" oncommand="Zotero_Long_Tag_Fixer.deselectAll()"/>
-					<button label="&zotero.general.selectAll;" oncommand="Zotero_Long_Tag_Fixer.selectAll()"/>
-				</hbox>
-			</vbox>
-			
-			<!-- Edit -->
-			<vbox>
-				<textbox id="zotero-new-tag-editor" multiline="true" flex="1"
-					oninput="Zotero_Long_Tag_Fixer.updateEditLength(this.value.length)"/>
-				<hbox id="zotero-new-tag-characters">
-					<label id="zotero-new-tag-character-count"/>
-					<label value="&zotero.sync.longTagFixer.characters;"/>
-				</hbox>
-			</vbox>
-			
-			<!-- Delete -->
-			<vbox align="center" pack="center">
-				<label value="&zotero.sync.longTagFixer.tagWillBeDeleted;"/>
-			</vbox>
-		</tabpanels>
-	</tabbox>
-</dialog>
diff --git a/chrome/content/zotero/xpcom/sync/syncRunner.js b/chrome/content/zotero/xpcom/sync/syncRunner.js
index 4ca1f8dbaf..fa0ef0aa5c 100644
--- a/chrome/content/zotero/xpcom/sync/syncRunner.js
+++ b/chrome/content/zotero/xpcom/sync/syncRunner.js
@@ -1214,62 +1214,54 @@ Zotero.Sync.Runner_Module = function (options = {}) {
 							   .getService(Components.interfaces.nsIWindowMediator);
 							var lastWin = wm.getMostRecentWindow("navigator:browser");
 							
-							// Open long tag fixer for every long tag in every editable library we're syncing
-							var editableLibraries = options.libraries
-								.filter(x => Zotero.Libraries.get(x).editable);
-							for (let libraryID of editableLibraries) {
-								let oldTagIDs = yield Zotero.Tags.getLongTagsInLibrary(libraryID);
-								for (let oldTagID of oldTagIDs) {
-									let oldTag = Zotero.Tags.getName(oldTagID);
-									let dataOut = { result: null };
-									lastWin.openDialog(
-										'chrome://zotero/content/longTagFixer.xul',
-										'',
-										'chrome,modal,centerscreen',
-										oldTag,
-										dataOut
-									);
-									// If dialog was cancelled, stop
-									if (!dataOut.result) {
-										return;
-									}
-									switch (dataOut.result.op) {
+							// Open long tag fixer for library we're syncing
+							let oldTagIDs = yield Zotero.Tags.getLongTagsInLibrary(object.libraryID);
+							
+							for (let oldTagID of oldTagIDs) {
+								let oldTag = Zotero.Tags.getName(oldTagID);
+								let dataOut = { result: null };
+								lastWin.openDialog(
+									'chrome://zotero/content/longTagFixer.xhtml',
+									'',
+									'chrome,modal,centerscreen',
+									{ oldTag, isLongTag: true },
+									dataOut
+								);
+								// If dialog was cancelled, stop
+								if (!dataOut.result) {
+									return;
+								}
+								const itemIDs = yield Zotero.Tags.getTagItems(object.libraryID, oldTagID);
+
+								switch (dataOut.result.op) {
 									case 'split':
-										for (let libraryID of editableLibraries) {
-											let itemIDs = yield Zotero.Tags.getTagItems(libraryID, oldTagID);
-											yield Zotero.DB.executeTransaction(async function () {
-												for (let itemID of itemIDs) {
-													let item = await Zotero.Items.getAsync(itemID);
-													for (let tag of dataOut.result.tags) {
-														item.addTag(tag);
-													}
-													item.removeTag(oldTag);
-													await item.save();
+										yield Zotero.DB.executeTransaction(async function () {
+											for (let itemID of itemIDs) {
+												let item = await Zotero.Items.getAsync(itemID);
+												let tagType = item.getTagType(oldTag);
+												for (let tag of dataOut.result.tags) {
+													item.addTag(tag, tagType);
 												}
-												await Zotero.Tags.purge(oldTagID);
-											});
-										}
+												item.removeTag(oldTag);
+												await item.save();
+											}
+											await Zotero.Tags.purge(oldTagID);
+										});
 										break;
 									
 									case 'edit':
-										for (let libraryID of editableLibraries) {
-											let itemIDs = yield Zotero.Tags.getTagItems(libraryID, oldTagID);
-											yield Zotero.DB.executeTransaction(async function () {
-												for (let itemID of itemIDs) {
-													let item = await Zotero.Items.getAsync(itemID);
-													item.replaceTag(oldTag, dataOut.result.tag);
-													await item.save();
-												}
-											});
-										}
+										yield Zotero.DB.executeTransaction(async function () {
+											for (let itemID of itemIDs) {
+												let item = await Zotero.Items.getAsync(itemID);
+												item.replaceTag(oldTag, dataOut.result.tag);
+												await item.save();
+											}
+										});
 										break;
 									
 									case 'delete':
-										for (let libraryID of editableLibraries) {
-											yield Zotero.Tags.removeFromLibrary(libraryID, oldTagID);
-										}
+										yield Zotero.Tags.removeFromLibrary(object.libraryID, oldTagID);
 										break;
-									}
 								}
 							}
 							
diff --git a/chrome/content/zotero/zoteroPane.xhtml b/chrome/content/zotero/zoteroPane.xhtml
index 458318386a..351dcb52fc 100644
--- a/chrome/content/zotero/zoteroPane.xhtml
+++ b/chrome/content/zotero/zoteroPane.xhtml
@@ -1202,6 +1202,8 @@
 								oncommand="ZoteroPane.tagSelector.openColorPickerWindow(); event.stopPropagation();"/>
 							<menuitem label="&zotero.tagSelector.renameTag;"
 								oncommand="ZoteroPane.tagSelector.openRenamePrompt(); event.stopPropagation();"/>
+							<menuitem id="split-tag" label="&zotero.tagSelector.splitTag;"
+								oncommand="ZoteroPane.tagSelector.openTagSplitterWindow(event)"/>
 							<menuitem label="&zotero.tagSelector.deleteTag;"
 								oncommand="ZoteroPane.tagSelector.openDeletePrompt(); event.stopPropagation();"/>
 						</menupopup>
diff --git a/chrome/locale/en-US/zotero/zotero.dtd b/chrome/locale/en-US/zotero/zotero.dtd
index e545cd8b95..e8e6b3de3e 100644
--- a/chrome/locale/en-US/zotero/zotero.dtd
+++ b/chrome/locale/en-US/zotero/zotero.dtd
@@ -139,6 +139,7 @@
 <!ENTITY zotero.tagSelector.assignColor              "Assign Color…">
 <!ENTITY zotero.tagSelector.renameTag				"Rename Tag…">
 <!ENTITY zotero.tagSelector.deleteTag				"Delete Tag…">
+<!ENTITY zotero.tagSelector.splitTag			"Split…">
 
 <!ENTITY zotero.tagColorChooser.title          "Choose a Tag Color and Position">
 <!ENTITY zotero.tagColorChooser.color          "Color:">
diff --git a/chrome/skin/default/zotero/longTagFixer.css b/chrome/skin/default/zotero/longTagFixer.css
deleted file mode 100644
index b8dde18916..0000000000
--- a/chrome/skin/default/zotero/longTagFixer.css
+++ /dev/null
@@ -1,8 +0,0 @@
-#zotero-new-tag-characters[invalid="true"] {
-	color: red;
-}
-
-#zotero-new-tag-character-count {
-	font-weight: bold;
-	margin-right: 0;
-}
diff --git a/scss/_zotero-react-client.scss b/scss/_zotero-react-client.scss
index f2b9f3ac97..bd0ceb0c9d 100644
--- a/scss/_zotero-react-client.scss
+++ b/scss/_zotero-react-client.scss
@@ -25,11 +25,14 @@
 @import "components/autosuggest";
 @import "components/button";
 @import "components/clicky";
+@import "components/collection-tree";
 @import "components/createParent";
 @import "components/dictionaryManager";
 @import "components/editable";
 @import "components/exportOptions";
 @import "components/icons";
+@import "components/item-tree";
+@import "components/longTagFixer";
 @import "components/mainWindow";
 @import "components/notesList";
 @import "components/progressMeter";
@@ -38,9 +41,7 @@
 @import "components/tabBar";
 @import "components/tagsBox";
 @import "components/tagSelector";
-@import "components/collection-tree";
 @import "components/virtualized-table";
-@import "components/item-tree";
 
 // Elements
 // --------------------------------------------------
diff --git a/scss/abstracts/_utilities.scss b/scss/abstracts/_utilities.scss
index d8e3164bb6..34d7b23067 100644
--- a/scss/abstracts/_utilities.scss
+++ b/scss/abstracts/_utilities.scss
@@ -2,3 +2,8 @@
 // Utilities
 // --------------------------------------------------
 
+@mixin text-truncate($text-overflow: ellipsis) {
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: $text-overflow;
+}
\ No newline at end of file
diff --git a/scss/components/_longTagFixer.scss b/scss/components/_longTagFixer.scss
new file mode 100644
index 0000000000..df0e01e17a
--- /dev/null
+++ b/scss/components/_longTagFixer.scss
@@ -0,0 +1,112 @@
+#zotero-long-tag-fixer {
+	min-width: 400px; // with intro disabled, dialog tends to get to narrow
+
+	tab {
+		display: none;
+		
+		@include variant("#zotero-long-tag-fixer.is-long-tag") {
+			display: revert;
+		}
+	}
+
+	tabpanels {
+		padding-top: 15px;
+
+		@include variant("#zotero-long-tag-fixer.is-long-tag") {
+			padding-top: 33px;
+		}
+	}
+
+	tab[visuallyselected="true"]:not(:-moz-window-inactive) {
+		color: unset;
+	}
+
+	#intro {
+		display: none;
+		flex-direction: column;
+
+		@include variant("#zotero-long-tag-fixer.is-long-tag") {
+			display: flex;
+		}
+
+		> label {
+			margin: .5em 0 0 0;
+		}
+
+		> textarea {
+			margin: 1em;
+			height: 50px;
+			border: none;
+			padding: .5em;
+			background-color: $shade-1;
+			appearance: none;
+			outline: none;
+		}
+	}
+
+	#zotero-new-tag-actions {
+		margin-top: 1em;
+	}
+
+	.split-tab {
+		.tag-list {
+			height: calc(8 * (1em + 10px));
+			overflow-x: hidden;
+			overflow-y: scroll;
+
+			> richlistitem {
+				display: block;
+				height: calc(1em + 10px);
+				overflow: hidden;
+			}
+
+			div {
+				display: flex;
+				align-items: center;
+			}
+
+			label  {
+				// width of the label is set to the width of the viewport minus sum of widths of
+				// paddings, margins and a checkbox to produce ellipsis truncation.
+				width: calc(100vw - 100px); 
+				@include text-truncate;
+			}
+		}
+
+		.delimiter-input {
+			width: 20px;
+		}
+
+		.delimiter-input-wrap {
+			display: flex;
+			align-items: center;
+		}
+
+		.tag-list + label {
+			padding: 0.5em 0;
+		}
+	}
+
+	.edit-tab {
+		display: flex;
+		flex-direction: column;
+
+		#zotero-new-tag-editor {
+			flex: 1 1 auto;
+		}
+
+		#zotero-new-tag-characters {
+			flex: 0 1 auto;
+			padding: .5em 0;
+		}
+	}
+
+	.invalid {
+		color: $red;
+	}
+
+	#zotero-new-tag-character-count {
+		font-weight: bold;
+		margin-right: 0;
+	}
+}
\ No newline at end of file
diff --git a/test/tests/syncRunnerTest.js b/test/tests/syncRunnerTest.js
index 6a5bd72746..86f24cf0bc 100644
--- a/test/tests/syncRunnerTest.js
+++ b/test/tests/syncRunnerTest.js
@@ -1321,7 +1321,7 @@ describe("Zotero.Sync.Runner", function () {
 					}
 				});
 				
-				waitForDialog(null, 'accept', 'chrome://zotero/content/longTagFixer.xul');
+				waitForDialog(null, 'accept', 'chrome://zotero/content/longTagFixer.xhtml');
 				yield runner.sync({ libraries: [Zotero.Libraries.userLibraryID] });
 				
 				assert.isFalse(Zotero.Tags.getID(tag));
@@ -1391,9 +1391,9 @@ describe("Zotero.Sync.Runner", function () {
 					}
 				});
 				
-				waitForDialog(function (dialog) {
-					dialog.Zotero_Long_Tag_Fixer.switchMode(2);
-				}, 'accept', 'chrome://zotero/content/longTagFixer.xul');
+				waitForDialog(function (window) {
+					window.Zotero_Long_Tag_Fixer.switchMode(2);
+				}, 'accept', 'chrome://zotero/content/longTagFixer.xhtml');
 				yield runner.sync({ libraries: [Zotero.Libraries.userLibraryID] });
 				
 				assert.isFalse(Zotero.Tags.getID(tag));