Scrolling in chat not working correctly with small height

The bug

When you set the Focused Height of the chat to something smaller than the length of a chat message, at some point scrolling does not show some lines anymore.

How to reproduce

1. Set the Focused Height of the chat to 20px
2. Use the following command

   /tellraw @p {"text":"1\n2\n3\n4\n5\n6\n7\n8\n9\n10\n11\n12\n13\n14\n15\n16\n17\n18\n19\n20"}
   

3. Try to scroll up to see number 18
   The chat scrolls to number 13