fix merge base scroll bug

Author: davidpdrsn

crates/but/src/command/legacy/status/tui/mod.rs

@@ -411,7 +411,12 @@ impl App {
 
     /// Returns the number of terminal rows available for rendering the status list.
     fn status_viewport_height(&self, terminal_area: Rect) -> usize {
-        usize::from(self.status_content_area(terminal_area).height).max(1)
+        let content_area = self.status_content_area(terminal_area);
+        let status_area = self.status_layout(content_area).status_area;
+
+        // The status pane uses a bottom border, so the inner list viewport is one row shorter
+        // than the outer area.
+        usize::from(status_area.height.saturating_sub(1)).max(1)
     }
 
     /// Returns the rendered height in terminal rows for the given status line.

crates/but/src/command/legacy/status/tui/tests/mod.rs

@@ -334,6 +334,23 @@ fn section_jumps_scroll_viewport_when_target_is_offscreen() {
         .assert_current_line_eq(str!["┊╭┄g0 [A]"]);
 }
 
+#[test]
+fn moving_to_merge_base_in_branch_mode_scrolls_to_keep_selection_visible() {
+    let env = Sandbox::init_scenario_with_target_and_default_settings("two-stacks").unwrap();
+    env.setup_metadata(&["A", "B"]).unwrap();
+
+    let mut tui = test_tui_with_size(env, 100, 8);
+
+    tui.input_then_render('b')
+        .assert_current_line_eq(str!["┊╭┄<< target >> g0 [A]"]);
+
+    tui.input_then_render((KeyModifiers::SHIFT, KeyCode::Char('J')))
+        .assert_current_line_eq(str!["┊╭┄<< target >> h0 [B]"]);
+
+    tui.input_then_render(KeyCode::Down)
+        .assert_current_line_eq(str!["[..]<< target >> [..] [origin/main] 2000-01-02 add M"]);
+}
+
 #[test]
 fn reload_preserves_visible_selection_when_scrolled() {
     let env = Sandbox::init_scenario_with_target_and_default_settings("two-stacks").unwrap();