1. Select a message.
2. Scroll >100 messages up.
3. Select another message.
4. Click the icon to select everything in between, selecting >100 messages
5. Click the forward icon.
6. Choose the chat and send.

Result: Nothing happens, because you can't forward more than 100 messages. But the mass-selection tool doesn't warn about that, only selecting messages individually does.
Expected result: Hiding the forward button when >100 messages are selected. Then the limit on manually selecting more could also be removed. Simply hiding the button would also prevent the issue of losing a big selection when trying to forward.