This change fixes a bug when a block is focused but offscreen.
Previously, we used the last row, but this caused a spurious block to be
rendered when scrolled to the end of the file. With this change we
always render off-screen blocks below the editor's clipping box.
Release Notes:
- Fixed a bug that caused the inline assistant to be displayed twice in
certain circumstances.
Co-authored-by: Nathan <nathan@zed.dev>