MarkdownDocument: blocks open on a read of the text as it is, top lines wait for their read and keep their place in tall blocks

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

src/documents/document/visual.rs

@@ -133,7 +133,9 @@ impl MarkdownDocument {
 }
 
 impl Render for MarkdownDocument {
-    fn render(&mut self, _: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
+    fn render(&mut self, window: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
+        // A top line asked before its text was read.
+        self.place_top(window, cx);
         let colors = cx.theme().colors.clone();
         let root = div()
             .size_full()