test(tui): cover Markdown drag-copy with regression tests (#6156)

Three focused tests in mouse_ui/primary_tests.rs: default-on Markdown
payload preserving fences with a cell-count toast, rendered-text
fallback when selection_copy_markdown=false, and multi-cell blank-line
join with count. Update
mouse_selection_autocopies_on_release_without_ctrl_c for the toast
receipt (clipboard assertion unchanged).

Validation: tui::mouse_ui::primary_tests 8 passed, 0 failed;
updated ui test 1 passed; updated ui test FAILS without the fix
(status-sink assertion), proving it pins the new behavior.

crates/tui/src/tui/mouse_ui/primary_tests.rs

@@ -209,3 +209,115 @@ fn transcript_drag_released_over_composer_keeps_caret_and_clears_drag() {
     assert_eq!(app.clipboard.read_primary_text(), Some(expected));
     assert_eq!(app.clipboard.last_written_text(), Some("REGULAR_CLIPBOARD"));
 }
+
+fn select_all_transcript_lines(app: &mut App) {
+    let last = app
+        .viewport
+        .transcript_cache
+        .lines()
+        .len()
+        .saturating_sub(1);
+    app.viewport.transcript_selection.anchor = Some(TranscriptSelectionPoint {
+        line_index: 0,
+        column: 0,
+    });
+    app.viewport.transcript_selection.head = Some(TranscriptSelectionPoint {
+        line_index: last,
+        column: 80,
+    });
+}
+
+fn ensure_transcript(app: &mut App) {
+    app.resync_history_revisions();
+    app.viewport.transcript_cache.ensure(
+        &app.history,
+        &app.history_revisions,
+        80,
+        app.transcript_render_options(),
+    );
+}
+
+#[test]
+fn transcript_copy_defaults_to_markdown_source_and_names_cell_count() {
+    let (mut app, _) = composer();
+    assert!(
+        app.viewport.selection_copy_markdown,
+        "markdown copy defaults on"
+    );
+    let content = "# Title\n\n```rust\nlet x = 1;\n```".to_string();
+    app.history = vec![HistoryCell::Assistant {
+        content: content.clone(),
+        streaming: false,
+    }];
+    ensure_transcript(&mut app);
+    select_all_transcript_lines(&mut app);
+    let toasts_before = app.status_toasts.len();
+    copy_active_selection(&mut app);
+    assert_eq!(app.clipboard.last_written_text(), Some(content.as_str()));
+    assert_eq!(app.status_toasts.len(), toasts_before + 1);
+    let toast = app.status_toasts.back().expect("markdown copy toast");
+    assert!(
+        toast.text.contains("(1)"),
+        "toast names the projected cell count, got {:?}",
+        toast.text
+    );
+    assert!(
+        app.status_message.is_none(),
+        "toast path must not touch the legacy status sink"
+    );
+}
+
+#[test]
+fn transcript_copy_falls_back_to_rendered_text_when_markdown_is_off() {
+    let (mut app, _) = composer();
+    app.viewport.selection_copy_markdown = false;
+    app.history = vec![HistoryCell::Assistant {
+        content: "# Title\n\n```rust\nlet x = 1;\n```".to_string(),
+        streaming: false,
+    }];
+    ensure_transcript(&mut app);
+    select_all_transcript_lines(&mut app);
+    let expected = selection_to_text(&app).expect("rendered fallback text");
+    let toasts_before = app.status_toasts.len();
+    copy_active_selection(&mut app);
+    assert_eq!(
+        app.clipboard.last_written_text(),
+        Some(expected.as_str())
+    );
+    assert_eq!(app.status_message.as_deref(), Some("Selection copied"));
+    assert_eq!(app.status_toasts.len(), toasts_before);
+}
+
+#[test]
+fn transcript_copy_joins_cells_with_blank_lines_and_counts_them() {
+    let (mut app, _) = composer();
+    app.history = vec![
+        HistoryCell::User {
+            content: "first question".into(),
+        },
+        HistoryCell::Assistant {
+            content: "second answer".into(),
+            streaming: false,
+        },
+    ];
+    ensure_transcript(&mut app);
+    select_all_transcript_lines(&mut app);
+    // A partial drag still rounds out to whole cells.
+    app.viewport.transcript_selection.anchor = Some(TranscriptSelectionPoint {
+        line_index: 0,
+        column: 2,
+    });
+    let toasts_before = app.status_toasts.len();
+    copy_active_selection(&mut app);
+    assert_eq!(
+        app.clipboard.last_written_text(),
+        Some("first question\n\nsecond answer")
+    );
+    assert_eq!(app.status_toasts.len(), toasts_before + 1);
+    let toast = app.status_toasts.back().expect("multi-cell copy toast");
+    assert!(
+        toast.text.contains("(2)"),
+        "toast names the projected cell count, got {:?}",
+        toast.text
+    );
+}

crates/tui/src/tui/ui/tests.rs

@@ -3655,7 +3655,19 @@ fn mouse_selection_autocopies_on_release_without_ctrl_c() {
         },
     );
 
-    assert_eq!(app.status_message.as_deref(), Some("Selection copied"));
+    // Drag-release autocopy now takes the Markdown-source path (#6156): the
+    // clipboard holds canonical cell content and the receipt is a toast
+    // naming the projected cell count, not the legacy status sink.
+    assert!(
+        app.status_message.is_none(),
+        "markdown copy must not touch the legacy status sink"
+    );
+    let toast = app.status_toasts.back().expect("markdown copy toast");
+    assert!(
+        toast.text.contains("(1)"),
+        "toast names the projected cell count, got {:?}",
+        toast.text
+    );
     assert!(
         app.clipboard
             .last_written_text()