The scroll bar should never flash for terminal tool output when content is added

We want to prevent the scroll bar from appearing when output is added or when it's initially expanded:

<img width="557" height="233" alt="Image" src="https://github.com/user-attachments/assets/38f04fa6-2a45-4dc5-9870-072f5aa2c565" />