diff --git a/Forms/GridField/GridFieldDetailForm_ItemRequest.php b/Forms/GridField/GridFieldDetailForm_ItemRequest.php
index 257630fd7..c512c0354 100644
--- a/Forms/GridField/GridFieldDetailForm_ItemRequest.php
+++ b/Forms/GridField/GridFieldDetailForm_ItemRequest.php
@@ -254,7 +254,7 @@ class GridFieldDetailForm_ItemRequest extends RequestHandler
 				'type' => 'Includes',
 				'SilverStripe\\Admin\\LeftAndMain_EditForm',
 			]);
-			$form->addExtraClass('cms-content cms-edit-form center');
+			$form->addExtraClass('cms-content cms-edit-form center flexbox-area-grow');
 			$form->setAttribute('data-pjax-fragment', 'CurrentForm Content');
 			if ($form->Fields()->hasTabSet()) {
 				$form->Fields()->findOrMakeTab('Root')->setTemplate('SilverStripe\\Forms\\CMSTabSet');
diff --git a/admin/client/dist/styles/bundle.css b/admin/client/dist/styles/bundle.css
index 9ac0da5f2..115767184 100644
--- a/admin/client/dist/styles/bundle.css
+++ b/admin/client/dist/styles/bundle.css
@@ -8623,6 +8623,10 @@ form.nostyle .TreeDropdownField,form.nostyle input.text,form.nostyle select,form
   max-width:auto;
 }
 
+fieldset{
+  width:100%;
+}
+
 .field.ui-tabs{
   border-bottom:0;
 }
diff --git a/admin/client/src/components/FieldHolder/FieldHolder.js b/admin/client/src/components/FieldHolder/FieldHolder.js
index 53a054ea9..ba9ef7616 100644
--- a/admin/client/src/components/FieldHolder/FieldHolder.js
+++ b/admin/client/src/components/FieldHolder/FieldHolder.js
@@ -60,7 +60,7 @@ function fieldHolder(Field) {
       // The extraClass property is defined on both the holder and element
       // for legacy reasons (same behaviour as PHP rendering)
       const classNames = [
-        'field form__field-holder',
+        'field',
         this.props.extraClass,
       ];
       if (this.props.readOnly) {
@@ -81,7 +81,9 @@ function fieldHolder(Field) {
       return (
         <FormGroup {...this.getHolderProps()}>
           {this.getLeftTitle()}
-          <Field {...this.props} />
+          <div className="form__field-holder">
+            <Field {...this.props} />
+          </div>
           {this.getRightTitle()}
           {this.getDescription()}
         </FormGroup>
diff --git a/admin/client/src/legacy/LeftAndMain.Preview.js b/admin/client/src/legacy/LeftAndMain.Preview.js
index 278feda96..4c56e6024 100644
--- a/admin/client/src/legacy/LeftAndMain.Preview.js
+++ b/admin/client/src/legacy/LeftAndMain.Preview.js
@@ -119,6 +119,8 @@ $.entwine('ss.preview', function($){
 
       if(save !== false) this.saveState('mode', modeName);
 
+      this.redraw();
+
       return this;
     },
 
@@ -289,6 +291,10 @@ $.entwine('ss.preview', function($){
       }
     },
 
+    onforcecontent: function () {
+      this.changeMode('content', false);
+    },
+
     onenable: function () {
       var $viewModeSelector = $('.preview-mode-selector');
 
diff --git a/admin/client/src/legacy/LeftAndMain.js b/admin/client/src/legacy/LeftAndMain.js
index b63e97113..e8f5c0608 100644
--- a/admin/client/src/legacy/LeftAndMain.js
+++ b/admin/client/src/legacy/LeftAndMain.js
@@ -336,15 +336,16 @@ $.entwine('ss', function($) {
         } else {
           preview.css('min-width', options.minPreviewWidth);
         }
-        $('.cms-preview').trigger('enable');
+        preview.trigger('enable');
       } else {
-        $('.cms-preview').trigger('disable');
+        preview.trigger('disable');
         if (mode == 'split') {
           // force change mode and leave it redraw after
-          this.contentViewMode();
+          preview.trigger('forcecontent');
           return true;
         }
       }
+
       this.addClass('cms-container--' + mode + '-mode');
       return false;
     },
diff --git a/admin/client/src/styles/legacy/_forms.scss b/admin/client/src/styles/legacy/_forms.scss
index 2f0e8d6e8..d51ccb64c 100644
--- a/admin/client/src/styles/legacy/_forms.scss
+++ b/admin/client/src/styles/legacy/_forms.scss
@@ -14,9 +14,10 @@ form.nostyle {
 	@include clear-form-field-styles();
 }
 
-// fieldset {
-// 	width: 100%;
-// }
+// need this for consistent <fieldset> behaviour
+fieldset {
+	width: 100%;
+}
 
 .field { // TODO convert to .form-group
 
diff --git a/admin/code/ModelAdmin.php b/admin/code/ModelAdmin.php
index f8fef5239..6071f1154 100644
--- a/admin/code/ModelAdmin.php
+++ b/admin/code/ModelAdmin.php
@@ -173,7 +173,7 @@ abstract class ModelAdmin extends LeftAndMain {
 			new FieldList($listField),
 			new FieldList()
 		)->setHTMLID('Form_EditForm');
-		$form->addExtraClass('cms-edit-form cms-panel-padded center');
+		$form->addExtraClass('cms-edit-form cms-panel-padded center flexbox-area-grow');
 		$form->setTemplate($this->getTemplatesWithSuffix('_EditForm'));
 		$editFormAction = Controller::join_links($this->Link($this->sanitiseClassName($this->modelClass)), 'EditForm');
 		$form->setFormAction($editFormAction);
diff --git a/admin/templates/SilverStripe/Admin/Includes/ModelAdmin_Content.ss b/admin/templates/SilverStripe/Admin/Includes/ModelAdmin_Content.ss
index 1a86d17e9..627261d14 100644
--- a/admin/templates/SilverStripe/Admin/Includes/ModelAdmin_Content.ss
+++ b/admin/templates/SilverStripe/Admin/Includes/ModelAdmin_Content.ss
@@ -1,4 +1,4 @@
-<div class="cms-content cms-tabset center $BaseCSSClasses" data-layout-type="border" data-pjax-fragment="Content">
+<div class="cms-content flexbox-area-grow cms-tabset center $BaseCSSClasses" data-layout-type="border" data-pjax-fragment="Content">
 
 	<div class="cms-content-header north">
 		<div class="cms-content-header-info">