OO-5180: Better calculation of the document height

e.g. course element title is respected

src/main/java/org/olat/course/nodes/document/ui/_content/run.html

@@ -26,11 +26,13 @@
 		<script>
 		/* <![CDATA[ */
 			b_resizeIframeToMainMaxHeight("cndContent");
-			var contentHeight = jQuery('#o_main_center_content').height();
+			var contentHeight = jQuery('#cndContent').height();
 			document.documentElement.style.setProperty('--cndcontentheight', `${contentHeight}px`);
 			jQuery(document).on("oo.window.resize.after", () => {
+				## reset height to a small value
+				jQuery('#cndContent').height('100px');
 				b_resizeIframeToMainMaxHeight("cndContent");
-				var contentHeight = jQuery('#o_main_center_content').height();
+				var contentHeight = jQuery('#cndContent').height();
 				document.documentElement.style.setProperty('--cndcontentheight', `${contentHeight}px`);
 			});
 		/* ]]> */

src/main/webapp/static/themes/light/modules/_doceditor.scss

@@ -48,7 +48,7 @@ body.o_doceditor_body {
 			iframe {
 				width: 100%;
 				height: 800px;        // Fallback if css variable not supported
-				height: calc(var(--cndcontentheight) - 90px);
+				height: calc(var(--cndcontentheight));
 				margin-top: 0px;
 				margin-bottom: 0px;
 				border-width: 1px;