There are two parts to showing the loading indicator:
1. The showLoadingIndicator property in the Teams manifest needs to be set to true.
2. When the tab loads, the app.initialize() and app.notifySuccess() methods of the Teams JS SDK v2 should be called to indicate to Teams that the app is ready to load and to hide the loading indicator.
Now there are some "interesting" things to know about this. If you are using the Teams Developer Portal to configure your Teams app, then as of now the showLoadingIndicator property is set to true by default and there is no way to change this in the portal. You will have to download the app manifest and make the necessary changes in the json manually.
If you are just starting with Microsoft Teams development and are unaware of the loading indicator option, the error message shown on the tab is not much help in figuring out the issue. It simply says:
"There was a problem reaching the app"
This is because the showLoadingIndicator property has been set to true by the Teams Developer portal and we don't yet have our tab calling the app.initialize() and app.notifySuccess() methods of the SDK. So, Teams thinks that our app has failed to load and shows the error message.
There are two possible solutions to fixing this problem:
1. Set the showLoadingIndicator property to false manually in the manifest json. But the drawback to this approach then, is that no native loading indicator would be shown, and the users might have to stare at a blank screen until the content loads.
2. Let the showLoadingIndicator property be set to true and then make sure we call the app.initialize() and app.notifySuccess() methods of the SDK.
Tab react code:
There are further options possible with the loading indicator like hiding the loading indicator even if the app continues to load in the background and explicitly specifying loading failure. To see more details about those, the Microsoft docs are a good place to start: Create a content page - Teams | Microsoft Learn
Hope this helps!