blubber: compute height without padding so that the textarea won't grow...

Closes #4413

Merge request studip/studip!3276

resources/vue/components/blubber/Comment.vue

@@ -121,8 +121,11 @@ export default {
         },
         setTextareaSize() {
             const textarea = this.$refs.textarea;
+            const style = getComputedStyle(textarea);
+            const height = textarea.scrollHeight - parseInt(style.paddingTop, 10) - parseInt(style.paddingBottom, 10);
+
             textarea.style.width = this.commentWidth + 'px';
-            textarea.style.height = textarea.scrollHeight + 'px';
+            textarea.style.height = height + 'px';
         }
     },
     mounted() {