Live: show disconnection error banner when in dev mode (#33394)

public/app/AppWrapper.tsx

@@ -13,6 +13,7 @@ import { SideMenu } from './core/components/sidemenu/SideMenu';
 import { GrafanaRoute } from './core/navigation/GrafanaRoute';
 import { AppNotificationList } from './core/components/AppNotifications/AppNotificationList';
 import { SearchWrapper } from 'app/features/search';
+import { LiveConnectionCorner } from './features/live/LiveConnectionCorner';
 
 interface AppWrapperProps {
   app: GrafanaApp;
@@ -112,6 +113,7 @@ export class AppWrapper extends React.Component<AppWrapperProps, AppWrapperState
                     </div>
                   </Router>
                 </div>
+                <LiveConnectionCorner />
                 <ModalRoot />
               </ModalsProvider>
             </ThemeProvider>

public/app/features/live/LiveConnectionCorner.tsx

@@ -0,0 +1,67 @@
+import { css } from '@emotion/css';
+import { GrafanaTheme } from '@grafana/data';
+import { config, getGrafanaLiveSrv } from '@grafana/runtime';
+import { stylesFactory } from '@grafana/ui';
+import React, { PureComponent } from 'react';
+import { Unsubscribable } from 'rxjs';
+
+export interface Props {}
+
+export interface State {
+  show?: boolean;
+}
+
+export class LiveConnectionCorner extends PureComponent<Props, State> {
+  subscription?: Unsubscribable;
+  styles = getStyle(config.theme);
+  state: State = {};
+
+  componentDidMount() {
+    // Only show the error in development mode
+    if (process.env.NODE_ENV === 'development') {
+      // Wait a second to listen for server errors
+      setTimeout(this.initListener, 1500);
+    }
+  }
+
+  initListener = () => {
+    const live = getGrafanaLiveSrv();
+    if (live) {
+      this.subscription = live.getConnectionState().subscribe({
+        next: (v) => {
+          this.setState({ show: !v });
+        },
+      });
+    }
+  };
+
+  componentWillUnmount() {
+    if (this.subscription) {
+      this.subscription.unsubscribe();
+    }
+  }
+
+  render() {
+    const { show } = this.state;
+    if (show) {
+      return <div className={this.styles.corner} title="The server is disconnected" />;
+    }
+    return null;
+  }
+}
+
+const getStyle = stylesFactory((theme: GrafanaTheme) => {
+  return {
+    corner: css`
+      position: fixed;
+      top: 0px !important;
+      left: 0px !important;
+      width: 0;
+      height: 0;
+      border-top: 60px solid ${theme.palette.brandWarning};
+      border-right: 60px solid transparent;
+      z-index: 10000;
+      cursor: wait;
+    `,
+  };
+});