feat: display error if app is not installed (coder#16980)

Fixes: coder#13937

Author: mtojek

site/src/modules/resources/AppLink/AppLink.stories.tsx

@@ -8,6 +8,7 @@ import {
 	MockWorkspaceApp,
 	MockWorkspaceProxies,
 } from "testHelpers/entities";
+import { withGlobalSnackbar } from "testHelpers/storybook";
 import { AppLink } from "./AppLink";
 
 const meta: Meta<typeof AppLink> = {
@@ -72,6 +73,19 @@ export const ExternalApp: Story = {
 	},
 };
 
+export const ExternalAppNotInstalled: Story = {
+	decorators: [withGlobalSnackbar],
+	args: {
+		workspace: MockWorkspace,
+		app: {
+			...MockWorkspaceApp,
+			external: true,
+			url: "foobar-foobaz://open-me",
+		},
+		agent: MockWorkspaceAgent,
+	},
+};
+
 export const SharingLevelOwner: Story = {
 	args: {
 		workspace: MockWorkspace,

site/src/modules/resources/AppLink/AppLink.tsx

@@ -5,7 +5,9 @@ import Link from "@mui/material/Link";
 import Tooltip from "@mui/material/Tooltip";
 import { API } from "api/api";
 import type * as TypesGen from "api/typesGenerated";
+import { displayError } from "components/GlobalSnackbar/utils";
 import { useProxy } from "contexts/ProxyContext";
+import { useEffect } from "react";
 import { type FC, type MouseEvent, useState } from "react";
 import { createAppLinkHref } from "utils/apps";
 import { generateRandomString } from "utils/random";
@@ -152,6 +154,20 @@ export const AppLink: FC<AppLinkProps> = ({ app, workspace, agent }) => {
 							url = href.replaceAll(magicTokenString, key.key);
 							setFetchingSessionToken(false);
 						}
+
+						// When browser recognizes the protocol and is able to navigate to the app,
+						// it will blur away, and will stop the timer. Otherwise,
+						// an error message will be displayed.
+						const openAppExternallyFailedTimeout = 500;
+						const openAppExternallyFailed = setTimeout(() => {
+							displayError(
+								`${app.display_name !== "" ? app.display_name : app.slug} must be installed first.`,
+							);
+						}, openAppExternallyFailedTimeout);
+						window.addEventListener("blur", () => {
+							clearTimeout(openAppExternallyFailed);
+						});
+
 						window.location.href = url;
 						return;
 					}