review-fix(computer_use): restore per-axis scroll coordinate semantics

main guarded scroll's x/y independently (coordinate=[null,100] -> y=100),
while click treats a coordinate without x as no point. The shared _xy()
applied click semantics to scroll; split out _scroll_xy() with main's
per-axis guard and pin both behaviors in a test.

tests/tools/test_computer_use.py

@@ -124,6 +124,18 @@ class TestDispatch:
         assert drag_kw["from_element"] == 1
         assert drag_kw["to_element"] == 5
 
+    def test_scroll_coordinate_axes_are_independent(self, noop_backend):
+        """scroll forwards each coordinate axis on its own (main parity):
+        ``coordinate=[null, 100]`` scrolls at y=100 with x=None, whereas
+        click treats a coordinate without x as no point at all."""
+        from tools.computer_use.tool import handle_computer_use
+        handle_computer_use({"action": "scroll", "coordinate": [None, 100]})
+        scroll_kw = next(c[1] for c in noop_backend.calls if c[0] == "scroll")
+        assert (scroll_kw["x"], scroll_kw["y"]) == (None, 100)
+        handle_computer_use({"action": "click", "coordinate": [None, 100]})
+        click_kw = next(c[1] for c in noop_backend.calls if c[0] == "click")
+        assert (click_kw["x"], click_kw["y"]) == (None, None)
+
 
     def test_capture_forwards_exact_pid_window_target(self, noop_backend):
         from tools.computer_use.tool import handle_computer_use

tools/computer_use/tool.py

@@ -292,8 +292,15 @@ def _summarize_action(action: str, args: Dict[str, Any]) -> str:
 #     final str/dict result. `delivery` = delivery_mode + bring_to_front; only input actions use it.
 
 def _xy(args: Dict[str, Any]) -> Dict[str, Any]:
+    """Click semantics: a coordinate only counts when its x is set (a bare y is not a point)."""
     return dict(x=coord[0], y=coord[1]) if (coord := args.get("coordinate")) and coord[0] is not None else dict(x=None, y=None)
 
+def _scroll_xy(args: Dict[str, Any]) -> Dict[str, Any]:
+    """Scroll semantics: axes are independent — ``coordinate=[null, 100]`` scrolls at y=100 with x unset."""
+    coord = args.get("coordinate") or (None, None)
+    return dict(x=coord[0] if coord and coord[0] is not None else None,
+                y=coord[1] if coord and coord[1] is not None else None)
+
 def _do_click(backend, action, args, button=None, count=1, **delivery):
     return backend.click(element=args.get("element"), **_xy(args), button=button or args.get("button") or "left",
                          click_count=count, modifiers=args.get("modifiers"), **delivery)
@@ -308,7 +315,7 @@ def _do_drag(backend, action, args, **delivery):
 
 def _do_scroll(backend, action, args, **delivery):
     return backend.scroll(direction=args.get("direction", "down"), amount=int(args.get("amount", 3)),
-                          element=args.get("element"), **_xy(args), modifiers=args.get("modifiers"), **delivery)
+                          element=args.get("element"), **_scroll_xy(args), modifiers=args.get("modifiers"), **delivery)
 
 def _do_capture(backend, action, args, **_):
     if (mode := str(args.get("mode", "som"))) not in {"som", "vision", "ax"}: