Resolve "Resize listener for IFrame in LegacyBaseTemplate does not trigger in some cases"

Closes #813 (closed)

Closes AlekSIS-App-Hjelp#51 (closed)

aleksis/core/frontend/components/LegacyBaseTemplate.vue

@@ -77,13 +77,13 @@ export default {
       const title = this.$refs.contentIFrame.contentWindow.document.title;
       this.$root.$setPageTitle(title);
 
-      // Adapt height of IFrame according to the height of its contents once and listen to resize events
+      // Adapt height of IFrame according to the height of its contents once and observe height changes
       this.iFrameHeight =
         this.$refs.contentIFrame.contentDocument.body.scrollHeight;
-      this.$refs.contentIFrame.contentWindow.onresize = () => {
+      new ResizeObserver(() => {
         this.iFrameHeight =
           this.$refs.contentIFrame.contentDocument.body.scrollHeight;
-      };
+      }).observe(this.$refs.contentIFrame.contentDocument.body)
 
       this.$root.contentLoading = false;
     },