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/mod.rs

@@ -97,6 +97,8 @@ pub struct MarkdownDocument {
     editing: bool,
     list: ListState,
     open: Option<Open>,
+    /// A dropped block's focus, for the document to take.
+    orphan: Option<FocusHandle>,
     /// A top line asked for, until the view holds it.
     top: Option<Placement>,
     asks: u64,
@@ -169,6 +171,7 @@ impl MarkdownDocument {
             editing: false,
             list: ListState::new(1, ListAlignment::Top, overdraw),
             open: None,
+            orphan: None,
             top: None,
             asks: 0,
             writes: 0,
@@ -344,7 +347,8 @@ impl MarkdownDocument {
 
     /// Closes the open block without writing it.
     fn drop_open(&mut self, cx: &mut Context<Self>) {
-        if self.open.take().is_some() {
+        if let Some(open) = self.open.take() {
+            self.orphan = Some(open.input.focus_handle(cx));
             log::info!("markdown document: the open block closed unwritten");
             cx.notify();
         }