tests: improved testability

tests/ui_snapshot.rs

@@ -0,0 +1,61 @@
+use ratatui::Terminal;
+use ratatui::backend::TestBackend;
+use ratatui::buffer::Buffer;
+use ratatui::layout::Rect;
+use ratatui::style::Style;
+use ratatui::widgets::{Block, Borders, List, ListItem, ListState};
+
+fn render_list_into_buffer(
+    lines: &[&str],
+    marked: &[bool],
+    selected: Option<usize>,
+    width: u16,
+    height: u16,
+) -> Buffer {
+    let backend = TestBackend::new(width.into(), height.into());
+    let mut terminal = Terminal::new(backend).expect("create terminal");
+
+    let lines_vec: Vec<ListItem> = lines
+        .iter()
+        .zip(marked.iter())
+        .map(|(text, marked)| {
+            let prefix = if *marked { 'x' } else { ' ' };
+            ListItem::new(format!("[{prefix}] {text}"))
+        })
+        .collect();
+
+    let mut state = ListState::default();
+    state.select(selected);
+
+    terminal
+        .draw(|f| {
+            let size = Rect::new(0, 0, width, height);
+
+            let list = List::new(lines_vec.clone())
+                .block(
+                    Block::default()
+                        .borders(Borders::ALL)
+                        .title("Lines (space to mark, q to quit)"),
+                )
+                .highlight_style(Style::default());
+
+            f.render_stateful_widget(list, size, &mut state);
+        })
+        .expect("draw");
+
+    // Extract the underlying backend buffer (clone to own it)
+    let backend = terminal.backend();
+    let buf = backend.buffer().clone();
+    buf
+}
+
+#[test]
+fn ui_snapshot_basic() {
+    let lines = ["first", "second", "third"];
+    let marked = [false, true, false];
+    let buf = render_list_into_buffer(&lines, &marked, Some(1), 40, 10);
+
+    // Use the buffer's debug representation for snapshotting
+    let out = format!("{buf:#?}");
+    insta::assert_snapshot!(out);
+}