what is the best way to tell users to scroll down in browsers?
Tests showed, that users do not scroll to the original message (which they need to answer questions):
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There's multiple ways to tell users to scroll, but the key is to prevent people from thinking it's the end of the page in the first place. Prevent a false bottom. Show a hint of the next segment, especially since you have a send action within the visible space. A lot of people will click here without checking the rest of the page, even if you do show a hint of the next segment. You may want to evaluate if you can change the hierarchy of the page.
If your design doesn't allow you to redo the layout so it's more obvious you can scroll, you can add an animated 'scroll' icon or pulsing arrow that points downward. These are more band aids than that they actually fix the issue though.