fix(tui): correct wheel/PageUp scroll direction and line-step

The scroll offset semantics were inverted for the wheel and PageUp:
is the content offset (larger = newer), so scrolling up must DECREASE it, but
ScrollUp and PageUp both INCREASED it — every wheel event (either direction)
moved toward the bottom, and at the bottom (follow=true) an up-notch added 3
then got clamped back to max, making the wheel dead until keyboard-scrolling
away. Now:

- wheel ScrollUp/ScrollLeft: offset -1, follow cleared (works from the bottom)
- wheel ScrollDown/ScrollRight: offset +1, follow restored at the bottom
- PageUp: offset - view_height; PageDown: offset + view_height
- one line per wheel notch (was 3) for line-by-line scrolling

regression test drives handle_mouse with synthetic events (direction,
line-step, no underflow, follow transitions).

crates/focus-tui/src/app.rs

@@ -707,9 +707,10 @@ impl App {
                 self.caret = self.line_end(self.caret);
             }
             KeyCode::PageUp => {
+                // 向上翻页：减小内容偏移（scroll 为内容偏移量）。
+                // Page up: decrease the content offset.
                 self.follow = false;
-                self.scroll = self.scroll.saturating_add(self.view_height.max(1));
+                self.scroll = self.scroll.saturating_sub(self.view_height.max(1));
-                self.scroll = self.scroll.min(self.max_scroll());
             }
             KeyCode::PageDown => {
                 self.scroll = (self.scroll + self.view_height.max(1)).min(self.max_scroll());
@@ -863,17 +864,20 @@ impl App {
                 }
             }
             // crossterm 把 Linux（xterm 协议）与 Windows（ConPTY）的滚轮事件
-            // 都归一化为 ScrollUp/ScrollDown；向上滚必须取消 follow，
+            // 都归一化为 ScrollUp/ScrollDown。注意方向：scroll 是内容偏移量，
-            // 否则每次绘制会被吸回底部。
+            // 向上滚（看更早内容）要减小偏移，向下滚才增大；每格 1 行，逐行滚动。
+            // 向上滚必须取消 follow，否则每次绘制会被吸回底部。
             // crossterm normalizes wheel events from both Linux (xterm) and
-            // Windows (ConPTY) to ScrollUp/ScrollDown; scrolling up must clear
+            // Windows (ConPTY) to ScrollUp/ScrollDown. Direction: `scroll` is
-            // follow, otherwise every draw snaps back to the bottom.
+            // the content offset, so scrolling up (older content) decreases
+            // it and scrolling down increases it; one line per notch.
+            // Scrolling up must clear follow, otherwise every draw snaps back.
             MouseEventKind::ScrollUp | MouseEventKind::ScrollLeft => {
                 self.follow = false;
-                self.scroll = self.scroll.saturating_add(3);
+                self.scroll = self.scroll.saturating_sub(1);
             }
             MouseEventKind::ScrollDown | MouseEventKind::ScrollRight => {
-                self.scroll = (self.scroll + 3).min(self.max_scroll());
+                self.scroll = (self.scroll + 1).min(self.max_scroll());
                 self.follow = self.scroll >= self.max_scroll();
             }
             _ => {}

crates/focus-tui/tests/app_session_tests.rs

@@ -1,6 +1,7 @@
 //! App 级会话持久化的测试（临时数据目录，零网络）。
 //! App-level session persistence tests (temp data dir, zero network).
 
+use crossterm::event::{KeyModifiers, MouseEvent, MouseEventKind};
 use focus_core::model::Message;
 use focus_harness::session::SessionStore;
 use focus_tui::app::App;
@@ -84,3 +85,59 @@ fn new_session_resets_state() {
     let sid2 = app.session_id.clone().unwrap();
     assert_ne!(sid1, sid2);
 }
+
+/// 滚轮方向与逐行滚动（回归：此前 ScrollUp/ScrollDown 方向写反，且贴底时
+/// 上滚被钳回底部导致滚轮失灵）。
+/// Wheel direction and line-by-line scrolling (regression: both wheel
+/// directions used to scroll down, and scrolling up at the bottom was clamped
+/// back, killing the wheel).
+#[test]
+fn wheel_scrolls_line_by_line_in_correct_direction() {
+    use focus_tui::app::BlockGeometry;
+    use focus_tui::state::BlockId;
+
+    let mut app = app_on(&temp_data_dir("wheel"));
+    // 模拟内容 100 行、视口 10 行的几何。
+    // Simulate geometry: 100 content lines, 10-line viewport.
+    app.geometry = vec![BlockGeometry {
+        id: BlockId("x".into()),
+        start: 0,
+        end: 100,
+    }];
+    app.view_height = 10;
+
+    // 贴底时向上滚：偏移减 1、取消 follow。
+    // At the bottom, scrolling up: offset -1, follow cleared.
+    app.scroll = 90;
+    app.follow = true;
+    app.handle_mouse(scroll_event(MouseEventKind::ScrollUp));
+    assert_eq!(app.scroll, 89, "wheel up must move up one line");
+    assert!(!app.follow);
+
+    // 向下滚：偏移 +1，逐行。
+    // Wheel down: offset +1, line by line.
+    app.handle_mouse(scroll_event(MouseEventKind::ScrollDown));
+    assert_eq!(app.scroll, 90);
+    assert!(app.follow, "reaching the bottom restores follow");
+
+    // 从顶部向下滚一次只动一行（不是直接跳到底部）。
+    // From the top, one notch moves exactly one line (no jump to the bottom).
+    app.scroll = 0;
+    app.follow = false;
+    app.handle_mouse(scroll_event(MouseEventKind::ScrollDown));
+    assert_eq!(app.scroll, 1);
+
+    // 顶部再向上滚不会下溢。
+    // Scrolling up at the top does not underflow.
+    app.handle_mouse(scroll_event(MouseEventKind::ScrollUp));
+    assert_eq!(app.scroll, 0);
+}
+
+fn scroll_event(kind: MouseEventKind) -> MouseEvent {
+    MouseEvent {
+        kind,
+        column: 0,
+        row: 0,
+        modifiers: KeyModifiers::NONE,
+    }
+}