Updates #1939: Enable smooth scrolling for short distances

sass/_core.scss

@@ -100,6 +100,10 @@ body.converse-fullscreen {
         background-color: #EEE !important;
     }
 
+    .smooth-scroll {
+        scroll-behavior: smooth !important;
+    }
+
     .nopadding {
         padding: 0 !important;
     }

src/converse-chatview.js

@@ -1360,6 +1360,11 @@ converse.plugins.add('converse-chatview', {
                     return;
                 }
                 if (u.isVisible(this.content) && !this.model.get('scrolled')) {
+                    if ((this.content.scrollTop === 0 || this.content.scrollTop < this.content.scrollHeight/2)) {
+                        u.removeClass('smooth-scroll', this.content);
+                    } else if (api.settings.get('animate')) {
+                        u.addClass('smooth-scroll', this.content);
+                    }
                     this.content.scrollTop = this.content.scrollHeight;
                 }
             },