clean up all the mess since commit fe4e1395d0

In the process we find a new bug. Scrolling with keyboard is overly eager to clamp screen_top to bottom of screen when the top used to be within the viewport.

Until recently, scrolling past the bottom when the margin was visible would move the cursor correctly but pan the surface to the top of the viewport. Slightly jarring, but good enough.

Created by  Kartik K. Agaram  on October 25, 2023
MU5W6A35USV3OPXIVGGY5FXC55KCIRQXOFHAGXMTOKCIL4MSQZ4AC
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