Chat sticky scroll: Don't see banner for most messages with smaller resized window

Testing #329727

- Send a few prompts in an editor window with small dimensions
- Try scrolling up and down
- I can get the banner to appear for the first message (screenshot below), but usually not anything past that

Maybe ultimately my ask is still the same as https://github.com/microsoft/vscode/issues/330010: I want to be able to see the banner at all times, not just under certain scrolling or height considerations.