diff --git a/wagtail/wagtailadmin/static_src/wagtailadmin/js/core.js b/wagtail/wagtailadmin/static_src/wagtailadmin/js/core.js
index 51f114e825..09dd2a1912 100644
--- a/wagtail/wagtailadmin/static_src/wagtailadmin/js/core.js
+++ b/wagtail/wagtailadmin/static_src/wagtailadmin/js/core.js
@@ -50,10 +50,6 @@ $(function() {
         fitNav();
     });
 
-    // Apply auto-height sizing to text areas
-    // NB .richtext (hallo.js-enabled) divs do not need this as they expand to fit their content by default
-    // $('.page-editor textarea').autosize();
-
     // Enable nice focus effects on all fields. This enables help text on hover.
     $(document).on('focus mouseover', 'input,textarea,select', function() {
         $(this).closest('.field').addClass('focused');
@@ -137,4 +133,12 @@ $(function() {
             });
         }
     }
+
+    /* Functions that need to run/rerun when active tabs are changed */
+    $(document).on('shown.bs.tab', function(e) {
+        // Resize autosize textareas
+        $('textarea[data-autosize-on]').each(function() {
+            autosize.update($(this).get());
+        });
+    });
 });
diff --git a/wagtail/wagtailadmin/static_src/wagtailadmin/js/vendor/jquery.autosize.js b/wagtail/wagtailadmin/static_src/wagtailadmin/js/vendor/jquery.autosize.js
index d0fa9645fc..053cf81555 100644
--- a/wagtail/wagtailadmin/static_src/wagtailadmin/js/vendor/jquery.autosize.js
+++ b/wagtail/wagtailadmin/static_src/wagtailadmin/js/vendor/jquery.autosize.js
@@ -1,242 +1,213 @@
 /*!
-	Autosize v1.17.2 - 2013-07-30
-	Automatically adjust textarea height based on user input.
-	(c) 2013 Jack Moore - http://www.jacklmoore.com/autosize
-	license: http://www.opensource.org/licenses/mit-license.php
+	Autosize 3.0.4
+	license: MIT
+	http://www.jacklmoore.com/autosize
 */
-(function (factory) {
+(function (global, factory) {
 	if (typeof define === 'function' && define.amd) {
-		// AMD. Register as an anonymous module.
-		define(['jquery'], factory);
+		define(['exports', 'module'], factory);
+	} else if (typeof exports !== 'undefined' && typeof module !== 'undefined') {
+		factory(exports, module);
 	} else {
-		// Browser globals: jQuery or jQuery-like library, such as Zepto
-		factory(window.jQuery || window.$);
+		var mod = {
+			exports: {}
+		};
+		factory(mod.exports, mod);
+		global.autosize = mod.exports;
 	}
-}(function ($) {
-	var
-	defaults = {
-		className: 'autosizejs',
-		append: '',
-		callback: false,
-		resizeDelay: 10
-	},
+})(this, function (exports, module) {
+	'use strict';
 
-	// border:0 is unnecessary, but avoids a bug in FireFox on OSX
-	copy = '<textarea tabindex="-1" style="position:absolute; top:-999px; left:0; right:auto; bottom:auto; border:0; -moz-box-sizing:content-box; -webkit-box-sizing:content-box; box-sizing:content-box; word-wrap:break-word; height:0 !important; min-height:0 !important; overflow:hidden; transition:none; -webkit-transition:none; -moz-transition:none;"/>',
+	function assign(ta) {
+		var _ref = arguments[1] === undefined ? {} : arguments[1];
 
-	// line-height is conditionally included because IE7/IE8/old Opera do not return the correct value.
-	typographyStyles = [
-		'fontFamily',
-		'fontSize',
-		'fontWeight',
-		'fontStyle',
-		'letterSpacing',
-		'textTransform',
-		'wordSpacing',
-		'textIndent'
-	],
+		var _ref$setOverflowX = _ref.setOverflowX;
+		var setOverflowX = _ref$setOverflowX === undefined ? true : _ref$setOverflowX;
+		var _ref$setOverflowY = _ref.setOverflowY;
+		var setOverflowY = _ref$setOverflowY === undefined ? true : _ref$setOverflowY;
 
-	// to keep track which textarea is being mirrored when adjust() is called.
-	mirrored,
+		if (!ta || !ta.nodeName || ta.nodeName !== 'TEXTAREA' || ta.hasAttribute('data-autosize-on')) {
+			return;
+		}var heightOffset = null;
+		var overflowY = 'hidden';
 
-	// the mirror element, which is used to calculate what size the mirrored element should be.
-	mirror = $(copy).data('autosize', true)[0];
+		function init() {
+			var style = window.getComputedStyle(ta, null);
 
-	// test that line-height can be accurately copied.
-	mirror.style.lineHeight = '99px';
-	if ($(mirror).css('lineHeight') === '99px') {
-		typographyStyles.push('lineHeight');
-	}
-	mirror.style.lineHeight = '';
+			if (style.resize === 'vertical') {
+				ta.style.resize = 'none';
+			} else if (style.resize === 'both') {
+				ta.style.resize = 'horizontal';
+			}
 
-	$.fn.autosize = function (options) {
-		options = $.extend({}, defaults, options || {});
+			if (style.boxSizing === 'content-box') {
+				heightOffset = -(parseFloat(style.paddingTop) + parseFloat(style.paddingBottom));
+			} else {
+				heightOffset = parseFloat(style.borderTopWidth) + parseFloat(style.borderBottomWidth);
+			}
 
-		if (mirror.parentNode !== document.body) {
-			$(document.body).append(mirror);
+			update();
 		}
 
-		return this.each(function () {
-			var
-			ta = this,
-			$ta = $(ta),
-			maxHeight,
-			minHeight,
-			boxOffset = 0,
-			callback = $.isFunction(options.callback),
-			originalStyles = {
-				height: ta.style.height,
-				overflow: ta.style.overflow,
-				overflowY: ta.style.overflowY,
-				wordWrap: ta.style.wordWrap,
-				resize: ta.style.resize
-			},
-			timeout,
-			width = $ta.width();
+		function changeOverflow(value) {
+			{
+				// Chrome/Safari-specific fix:
+				// When the textarea y-overflow is hidden, Chrome/Safari do not reflow the text to account for the space
+				// made available by removing the scrollbar. The following forces the necessary text reflow.
+				var width = ta.style.width;
+				ta.style.width = '0px';
+				// Force reflow:
+				/* jshint ignore:start */
+				ta.offsetWidth;
+				/* jshint ignore:end */
+				ta.style.width = width;
+			}
 
-			if ($ta.data('autosize')) {
-				// exit if autosize has already been applied, or if the textarea is the mirror element.
+			overflowY = value;
+
+			if (setOverflowY) {
+				ta.style.overflowY = value;
+			}
+
+			update();
+		}
+
+		function update() {
+			var startHeight = ta.style.height;
+			var htmlTop = document.documentElement.scrollTop;
+			var bodyTop = document.body.scrollTop;
+			var originalHeight = ta.style.height;
+
+			ta.style.height = 'auto';
+
+			var endHeight = ta.scrollHeight + heightOffset;
+
+			if (ta.scrollHeight === 0) {
+				// If the scrollHeight is 0, then the element probably has display:none or is detached from the DOM.
+				ta.style.height = originalHeight;
 				return;
 			}
-			$ta.data('autosize', true);
 
-			if ($ta.css('box-sizing') === 'border-box' || $ta.css('-moz-box-sizing') === 'border-box' || $ta.css('-webkit-box-sizing') === 'border-box'){
-				boxOffset = $ta.outerHeight() - $ta.height();
-			}
+			ta.style.height = endHeight + 'px';
 
-			// IE8 and lower return 'auto', which parses to NaN, if no min-height is set.
-			minHeight = Math.max(parseInt($ta.css('minHeight'), 10) - boxOffset || 0, $ta.height());
+			// prevents scroll-position jumping
+			document.documentElement.scrollTop = htmlTop;
+			document.body.scrollTop = bodyTop;
 
-			$ta.css({
-				overflow: 'hidden',
-				overflowY: 'hidden',
-				wordWrap: 'break-word', // horizontal overflow is hidden, so break-word is necessary for handling words longer than the textarea width
-				resize: ($ta.css('resize') === 'none' || $ta.css('resize') === 'vertical') ? 'none' : 'horizontal'
-			});
+			var style = window.getComputedStyle(ta, null);
 
-			function initMirror() {
-				var styles = {}, ignore;
-
-				mirrored = ta;
-				mirror.className = options.className;
-				maxHeight = parseInt($ta.css('maxHeight'), 10);
-
-				// mirror is a duplicate textarea located off-screen that
-				// is automatically updated to contain the same text as the
-				// original textarea.  mirror always has a height of 0.
-				// This gives a cross-browser supported way getting the actual
-				// height of the text, through the scrollTop property.
-				$.each(typographyStyles, function(i,val){
-					styles[val] = $ta.css(val);
-				});
-				$(mirror).css(styles);
-
-				// The textarea overflow is probably now hidden, but Chrome doesn't reflow the text to account for the
-				// new space made available by removing the scrollbars. This workaround causes Chrome to reflow the text.
-				if ('oninput' in ta) {
-					var width = ta.style.width;
-					ta.style.width = '0px';
-					ignore = ta.offsetWidth; // This value isn't used, but getting it triggers the necessary reflow
-					ta.style.width = width;
-				}
-			}
-
-			// Using mainly bare JS in this function because it is going
-			// to fire very often while typing, and needs to very efficient.
-			function adjust() {
-				var height, original, width, style;
-
-				if (mirrored !== ta) {
-					initMirror();
-				}
-
-				mirror.value = ta.value + options.append;
-				mirror.style.overflowY = ta.style.overflowY;
-				original = parseInt(ta.style.height,10);
-
-				// window.getComputedStyle, getBoundingClientRect returning a width are unsupported in IE8 and lower.
-				// The mirror width must exactly match the textarea width, so using getBoundingClientRect because it doesn't round the sub-pixel value.
-				if ('getComputedStyle' in window) {
-					style = window.getComputedStyle(ta);
-					width = ta.getBoundingClientRect().width;
-
-					$.each(['paddingLeft', 'paddingRight', 'borderLeftWidth', 'borderRightWidth'], function(i,val){
-						width -= parseInt(style[val],10);
-					});
-
-					mirror.style.width = width + 'px';
-				}
-				else {
-					mirror.style.width = Math.max($ta.width(), 0) + 'px';
-				}
-
-				// Needed for IE8 and lower to reliably return the correct scrollTop
-				mirror.scrollTop = 0;
-
-				mirror.scrollTop = 9e4;
-
-				// Using scrollTop rather than scrollHeight because scrollHeight is non-standard and includes padding.
-				height = mirror.scrollTop;
-
-				if (maxHeight && height > maxHeight) {
-					ta.style.overflowY = 'scroll';
-					height = maxHeight;
-				} else {
-					ta.style.overflowY = 'hidden';
-					if (height < minHeight) {
-						height = minHeight;
-					}
-				}
-
-				height += boxOffset;
-
-				if (original !== height) {
-					ta.style.height = height + 'px';
-					if (callback) {
-						options.callback.call(ta,ta);
-					}
-				}
-			}
-
-			function resize () {
-				clearTimeout(timeout);
-				timeout = setTimeout(function(){
-					if ($ta.width() !== width) {
-						adjust();
-					}
-				}, parseInt(options.resizeDelay,10));
-			}
-
-			if ('onpropertychange' in ta) {
-				if ('oninput' in ta) {
-					// Detects IE9.  IE9 does not fire onpropertychange or oninput for deletions,
-					// so binding to onkeyup to catch most of those occasions.  There is no way that I
-					// know of to detect something like 'cut' in IE9.
-					$ta.on('input.autosize keyup.autosize', adjust);
-				} else {
-					// IE7 / IE8
-					$ta.on('propertychange.autosize', function(){
-						if(event.propertyName === 'value'){
-							adjust();
-						}
-					});
+			if (style.height !== ta.style.height) {
+				if (overflowY !== 'visible') {
+					changeOverflow('visible');
+					return;
 				}
 			} else {
-				// Modern Browsers
-				$ta.on('input.autosize', adjust);
+				if (overflowY !== 'hidden') {
+					changeOverflow('hidden');
+					return;
+				}
 			}
 
-			// Set options.resizeDelay to false if using fixed-width textarea elements.
-			// Uses a timeout and width check to reduce the amount of times adjust needs to be called after window resize.
-
-			if (options.resizeDelay !== false) {
-				$(window).on('resize.autosize', resize);
+			if (startHeight !== ta.style.height) {
+				var evt = document.createEvent('Event');
+				evt.initEvent('autosize:resized', true, false);
+				ta.dispatchEvent(evt);
 			}
+		}
 
-			// Event for manual triggering if needed.
-			// Should only be needed when the value of the textarea is changed through JavaScript rather than user input.
-			$ta.on('autosize.resize', adjust);
+		var destroy = (function (style) {
+			window.removeEventListener('resize', update);
+			ta.removeEventListener('input', update);
+			ta.removeEventListener('keyup', update);
+			ta.removeAttribute('data-autosize-on');
+			ta.removeEventListener('autosize:destroy', destroy);
 
-			// Event for manual triggering that also forces the styles to update as well.
-			// Should only be needed if one of typography styles of the textarea change, and the textarea is already the target of the adjust method.
-			$ta.on('autosize.resizeIncludeStyle', function() {
-				mirrored = null;
-				adjust();
+			Object.keys(style).forEach(function (key) {
+				ta.style[key] = style[key];
 			});
+		}).bind(ta, {
+			height: ta.style.height,
+			resize: ta.style.resize,
+			overflowY: ta.style.overflowY,
+			overflowX: ta.style.overflowX,
+			wordWrap: ta.style.wordWrap });
 
-			$ta.on('autosize.destroy', function(){
-				mirrored = null;
-				clearTimeout(timeout);
-				$(window).off('resize', resize);
-				$ta
-					.off('autosize')
-					.off('.autosize')
-					.css(originalStyles)
-					.removeData('autosize');
-			});
+		ta.addEventListener('autosize:destroy', destroy);
 
-			// Call adjust in case the textarea already contains text.
-			adjust();
-		});
-	};
-}));
\ No newline at end of file
+		// IE9 does not fire onpropertychange or oninput for deletions,
+		// so binding to onkeyup to catch most of those events.
+		// There is no way that I know of to detect something like 'cut' in IE9.
+		if ('onpropertychange' in ta && 'oninput' in ta) {
+			ta.addEventListener('keyup', update);
+		}
+
+		window.addEventListener('resize', update);
+		ta.addEventListener('input', update);
+		ta.addEventListener('autosize:update', update);
+		ta.setAttribute('data-autosize-on', true);
+
+		if (setOverflowY) {
+			ta.style.overflowY = 'hidden';
+		}
+		if (setOverflowX) {
+			ta.style.overflowX = 'hidden';
+			ta.style.wordWrap = 'break-word';
+		}
+
+		init();
+	}
+
+	function destroy(ta) {
+		if (!(ta && ta.nodeName && ta.nodeName === 'TEXTAREA')) {
+			return;
+		}var evt = document.createEvent('Event');
+		evt.initEvent('autosize:destroy', true, false);
+		ta.dispatchEvent(evt);
+	}
+
+	function update(ta) {
+		if (!(ta && ta.nodeName && ta.nodeName === 'TEXTAREA')) {
+			return;
+		}var evt = document.createEvent('Event');
+		evt.initEvent('autosize:update', true, false);
+		ta.dispatchEvent(evt);
+	}
+
+	var autosize = null;
+
+	// Do nothing in IE8 or lower
+	if (typeof window.getComputedStyle !== 'function') {
+		autosize = function (el) {
+			return el;
+		};
+		autosize.destroy = function (el) {
+			return el;
+		};
+		autosize.update = function (el) {
+			return el;
+		};
+	} else {
+		autosize = function (el, options) {
+			if (el) {
+				Array.prototype.forEach.call(el.length ? el : [el], function (x) {
+					return assign(x, options);
+				});
+			}
+			return el;
+		};
+		autosize.destroy = function (el) {
+			if (el) {
+				Array.prototype.forEach.call(el.length ? el : [el], destroy);
+			}
+			return el;
+		};
+		autosize.update = function (el) {
+			if (el) {
+				Array.prototype.forEach.call(el.length ? el : [el], update);
+			}
+			return el;
+		};
+	}
+
+	module.exports = autosize;
+});
\ No newline at end of file
diff --git a/wagtail/wagtailadmin/widgets.py b/wagtail/wagtailadmin/widgets.py
index 825d825819..58e1f9eebc 100644
--- a/wagtail/wagtailadmin/widgets.py
+++ b/wagtail/wagtailadmin/widgets.py
@@ -24,7 +24,7 @@ class AdminAutoHeightTextInput(WidgetWithScript, widgets.Textarea):
         super(AdminAutoHeightTextInput, self).__init__(default_attrs)
 
     def render_js_init(self, id_, name, value):
-        return '$("#{0}").autosize();'.format(id_)
+        return 'autosize($("#{0}"));'.format(id_)
 
 class AdminDateInput(WidgetWithScript, widgets.DateInput):
     # Set a default date format to match the one that our JS date picker expects -