Cleanup timeout before unmount.

src/components/views/messages/MImageBody.js

@@ -204,6 +204,12 @@ export default class extends React.Component {
         this.unmounted = true;
         dis.unregister(this.dispatcherRef);
         this.context.matrixClient.removeListener('sync', this.onClientSync);
+        this._afterComponentWillUnmount();
+    }
+
+    // To be overridden by subclasses (e.g. MStickerBody) for further
+    // cleanup after componentWillUnmount
+    _afterComponentWillUnmount() {
     }
 
     onAction(payload) {

src/components/views/messages/MStickerBody.js

@@ -43,12 +43,13 @@ export default class MStickerBody extends MImageBody {
         this.setState({
             placeholderClasses: 'mx_MStickerBody_placeholder_invisible',
         });
-        setTimeout(() => {
+        const hidePlaceholderTimer = setTimeout(() => {
             this.setState({
                 placeholderVisible: false,
                 thumbnailClasses: 'mx_MStickerBody_thumbnail_visible',
             });
         }, 500);
+        this.setState({hidePlaceholderTimer});
     }
 
     _afterComponentDidMount() {
@@ -69,6 +70,13 @@ export default class MStickerBody extends MImageBody {
         }
     }
 
+    _afterComponentWillUnmount() {
+        if (this.state.hidePlaceholderTimer) {
+            clearTimeout(this.state.hidePlaceholderTimer);
+            this.setState({hidePlaceholderTimer: null});
+        }
+    }
+
     _messageContent(contentUrl, thumbUrl, content) {
         let tooltip;
         const tooltipBody = (