diff --git a/lib/editor/tinymce/plugins/pdw/readme_moodle.txt b/lib/editor/tinymce/plugins/pdw/readme_moodle.txt
index cf2fcb0a783..17f75168cd9 100644
--- a/lib/editor/tinymce/plugins/pdw/readme_moodle.txt
+++ b/lib/editor/tinymce/plugins/pdw/readme_moodle.txt
@@ -15,3 +15,4 @@ Upgrade procedure:
 5/ reimplement patch in MDL-40668
 6/ add in "DOM.setStyle(ifr, 'width',DOM.getSize(ifrcon).w); // Resize iframe" (without quotes)
    after "DOM.setStyle(ifr, 'height',DOM.getSize(ifr).h + dy); // Resize iframe"
+7/ reimplement patch in MDL-42481
diff --git a/lib/editor/tinymce/plugins/pdw/tinymce/editor_plugin.js b/lib/editor/tinymce/plugins/pdw/tinymce/editor_plugin.js
index 869114775f5..22e42b796ba 100644
--- a/lib/editor/tinymce/plugins/pdw/tinymce/editor_plugin.js
+++ b/lib/editor/tinymce/plugins/pdw/tinymce/editor_plugin.js
@@ -161,10 +161,27 @@
 
             // For very small text areas - allow the editable region to be smaller than the size of the toolbars.
             if (rows >= 3) {
+                // This set of changes addresses MDL-42481.
+                // Moodle collapses form sections by setting display to none on the fcontainer element.
+                // In order to calculate the offsetWidth, the iframe must be visible within the DOM, otherwise it's offsetWidth is
+                // calculate as 0px.
+                // We attempt to find any collapsed element, uncollapse them, then calculate the width and height, and finally
+                // collapse them again.
+                var collapsedContainer = DOM.getParent(ifr, 'fieldset.collapsed');
+                if (collapsedContainer) {
+                    DOM.removeClass(collapsedContainer, 'collapsed');
+                }
+
                 DOM.setStyle(ifr, 'height',DOM.getSize(ifr).h + dy); // Resize iframe
                 DOM.setStyle(ifr, 'width',DOM.getSize(ifrcon).w); // Resize iframe
                 ed.theme.deltaHeight += dy; // For resize cookie
+
+                if (collapsedContainer) {
+                    // We have a collapsedContainer, so collapse it again.
+                    DOM.addClass(collapsedContainer, 'collapsed');
+                }
             }
+
 		},
 
 		/**