Opening a chat on Snapchat does not trigger the "typing" notification. Your friends will only receive a notification that you are typing when you start typing a message. When you open a chat, your Bitmoji will appear in the chat, but no typing notification will be sent until you begin typing a message(source: source).
It's worth noting that sometimes, the dialog box may activate automatically when you open a chat, leading Snapchat to believe you are typing even if you are just reading the chat. While this can be a bit glitchy, most users are aware of this issue(source: source).
To avoid showing that you're typing when only opening a chat, you can try disconnecting your internet before opening the chat. Once you have checked the chat, you can close the app and remove it from your recent tabs to prevent the other user from being notified of any activity in the chat(source: source).
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