fix very long processing time in scroll handler

src/views/messages-view.jsx

@@ -185,14 +185,20 @@ class MessagesView extends React.Component {
 
   getVisibleMessageRange(holderRect) {
     let min = Number.MAX_SAFE_INTEGER, max = -1;
-    this.chatMessageRefs.forEach((ref, seq) => {
+    let visibilityStatus = false;
+    this.chatMessageRefs.every((ref, seq) => {
       if (ref.current) {
         const { top, bottom, height } = ref.current.getBoundingClientRect();
         const visible = top <= holderRect.top ? holderRect.top - top <= height : bottom - holderRect.bottom <= height;
         if (visible) {
+          visibilityStatus = true;
           min = Math.min(min, seq);
           max = Math.max(max, seq);
+        } else if (visibilityStatus) {
+          // The remaining elements are no longer visible, no need to iterate them.
+          return false;
         }
+        return true;
       }
     });
     return max >= min ? {min: min, max: max} : {min: 0, max: 0};
@@ -577,13 +583,14 @@ class MessagesView extends React.Component {
       showGoToLastButton: (pos > SHOW_GO_TO_LAST_DIST) && (pos < this.state.scrollPosition),
     });
 
-    if (this.state.fetchingMessages) {
+    if (this.state.fetchingMessages || this.processingScrollEvent) {
       return;
     }
 
     if (event.target.scrollTop <= FETCH_PAGE_TRIGGER) {
       const topic = this.props.tinode.getTopic(this.state.topic);
       if (topic && topic.isSubscribed()) {
+        this.processingScrollEvent = true;
         const {min, max} = this.getVisibleMessageRange(event.target.getBoundingClientRect());
         const gaps = topic.msgHasMoreMessages(min, max, false);
         if (gaps.length > 0) {
@@ -595,6 +602,7 @@ class MessagesView extends React.Component {
         }
       }
     }
+    this.processingScrollEvent = false;
   }
 
   /* Mount drag and drop events */