documents: undo in an untouched block steps the document; visual after preview opens an empty end

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

src/documents/document/tests/mod.rs

@@ -425,3 +425,50 @@ fn focus_opens_no_block_in_a_written_document(cx: &mut TestAppContext) {
     cx.simulate_input("x");
     assert_eq!(text(&document, cx), TEXT);
 }
+
+fn mode(document: &Entity<MarkdownDocument>, mode: MarkdownMode, cx: &mut VisualTestContext) {
+    document.update_in(cx, |document, window, cx| {
+        document.set_mode(mode, window, cx)
+    });
+    cx.run_until_parked();
+}
+
+#[gpui::test]
+fn preview_and_back_still_take_typing_in_an_empty_document(cx: &mut TestAppContext) {
+    let (document, cx) = open("", MarkdownMode::Visual, cx);
+    focus(&document, cx);
+    cx.simulate_keystrokes("escape");
+    mode(&document, MarkdownMode::Preview, cx);
+    mode(&document, MarkdownMode::Visual, cx);
+    cx.simulate_input("Hi");
+    assert_eq!(text(&document, cx), "Hi");
+}
+
+#[gpui::test]
+fn undo_in_an_untouched_block_steps_the_document(cx: &mut TestAppContext) {
+    let (document, cx) = open("Seed", MarkdownMode::Source, cx);
+    document.update(cx, |document, cx| document.set_text("", cx));
+    mode(&document, MarkdownMode::Visual, cx);
+    cx.simulate_keystrokes("secondary-z");
+    cx.run_until_parked();
+    assert_eq!(text(&document, cx), "Seed", "the source edit is undone");
+    cx.simulate_keystrokes("secondary-shift-z");
+    cx.run_until_parked();
+    assert_eq!(text(&document, cx), "", "and redone");
+}
+
+#[gpui::test]
+fn undo_in_a_typed_block_stays_in_the_block(cx: &mut TestAppContext) {
+    let (document, cx) = open("Seed", MarkdownMode::Source, cx);
+    document.update(cx, |document, cx| document.set_text(TEXT, cx));
+    mode(&document, MarkdownMode::Visual, cx);
+    press(1, Modifiers::none(), cx);
+    cx.simulate_input(" x");
+    cx.simulate_keystrokes("secondary-z");
+    cx.run_until_parked();
+    assert_eq!(
+        text(&document, cx),
+        TEXT,
+        "the typing went, not the edit before"
+    );
+}