Documents: an open block keeps its look

The open field sits in the closed block's frame at its size, line
height and weight; headings keep their step. Code and math keep their
panel. Typewriter centering also keys on the ends' room, so a block
opened at the same height still centers.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/documents/document/look.rs

@@ -90,9 +90,9 @@ impl MarkdownDocument {
             block.size.height -= self.room;
         }
         let view = self.list.viewport_bounds();
-        // A new layout or a resize moves it too.
-        let heights =
-            [block.size.height, view.size.height].map(|height| f32::from(height).round() as u64);
+        // A new layout, a resize or the ends' room moves it.
+        let heights = [block.size.height, view.size.height, self.room]
+            .map(|height| f32::from(height).round() as u64);
         let moved = (ix, self.edits, self.typed, heights);
         if self.centered == Some(moved) {
             return;