diff --git a/chrome/content/scaffold/scaffold.xhtml b/chrome/content/scaffold/scaffold.xhtml
index c5d1b13d50..6faa6831ed 100644
--- a/chrome/content/scaffold/scaffold.xhtml
+++ b/chrome/content/scaffold/scaffold.xhtml
@@ -468,9 +468,7 @@
 					</tabpanel>
 					<tabpanel flex="1" id="tabpanel-tests">
 						<vbox flex="1" style="max-width: 100%;">
-							<!-- The `height` attribute on the vbox and iframe is overridden by `flex`. Without it,
-							     though, the richlistbox grows when tests are added. -->
-							<vbox flex="1" height="auto">
+							<vbox id="testing-listbox-container">
 								<keyset>
 									<key id="key-delete-tests" observes="validate-tests" keycode="VK_BACK" oncommand="Scaffold.deleteSelectedTests()"/>
 								</keyset>
@@ -505,7 +503,7 @@
 								</hbox>
 							</vbox>
 							<splitter/>
-							<iframe src="monaco/monaco.html" id="editor-tests" flex="2" height="auto" onmousedown="this.focus()"/>
+							<iframe src="monaco/monaco.html" id="editor-tests" onmousedown="this.focus()"/>
 						</vbox>
 					</tabpanel>
 					<tabpanel>
diff --git a/scss/scaffold.scss b/scss/scaffold.scss
index cef4c7d837..adf34761e0 100644
--- a/scss/scaffold.scss
+++ b/scss/scaffold.scss
@@ -172,3 +172,11 @@ vbox > splitter {
 		height: 300px;
 	}
 }
+
+#testing-listbox-container {
+	flex: 1;
+}
+
+#editor-tests {
+	flex: 2;
+}