documents: a dropped block hands its focus to the document

An edit past an open block wrote it back and dropped its input while
it held focus, so focus fell to the window root and Undo reached
nothing. The next render passes such focus to the document.

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

src/documents/document/visual.rs

@@ -195,6 +195,7 @@ impl MarkdownDocument {
 
 impl Render for MarkdownDocument {
     fn render(&mut self, window: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
+        self.take_orphan(window, cx);
         // A top line asked before its text was read.
         self.place_top(window, cx);
         self.show_waiting(window, cx);