A short session showed the bottom-anchor's sub-row slack as a visible
gap above the first line. The grid now stays top-anchored like a fresh
terminal until scrollback exists — the point where the prompt actually
lives on the bottom row and resize stability matters. The origin is
recomputed in the render frame (covering both resize- and
content-driven flips) and still only moves together with a forced full
repaint.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>