MM-53147 Support for embedding Mattermost in an MSTeams iframe (#23776)

* add teams to allowed frame-ancestors

* fix unit tests

* set SameSite attribute for session cookie

* further restrict ancestors

* skip landing page if in iframe

* Only set cookie SameSite=None if embedded in iframe

* don't set MMEMBED cookie on landing page (check only)

* fully parse MMEMBED cookie

* add comment

* more comments

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

webapp/channels/src/components/linking_landing_page/linking_landing_page.tsx

@@ -79,6 +79,20 @@ export default class LinkingLandingPage extends PureComponent<Props, State> {
         return landingPreference && landingPreference === LandingPreferenceTypes.BROWSER;
     };
 
+    isEmbedded = () => {
+        // this cookie is set by any plugin that facilitates iframe embedding (e.g. mattermost-plugin-msteams-sync).
+        const cookieName = 'MMEMBED';
+        const cookies = document.cookie.split(';');
+        for (let i = 0; i < cookies.length; i++) {
+            const cookie = cookies[i].trim();
+            if (cookie.startsWith(cookieName + '=')) {
+                const value = cookie.substring(cookieName.length + 1);
+                return decodeURIComponent(value) === '1';
+            }
+        }
+        return false;
+    };
+
     checkLandingPreferenceApp = () => {
         const landingPreference = BrowserStore.getLandingPreference(this.props.siteUrl);
         return landingPreference && landingPreference === LandingPreferenceTypes.MATTERMOSTAPP;
@@ -435,7 +449,7 @@ export default class LinkingLandingPage extends PureComponent<Props, State> {
     render() {
         const isMobile = UserAgent.isMobile();
 
-        if (this.checkLandingPreferenceBrowser()) {
+        if (this.checkLandingPreferenceBrowser() || this.isEmbedded()) {
             this.openInBrowser();
             return null;
         }