fix: canvasCoords crashes on empty touch list (mobile race) (#2045)

tests/test_canvas_coords_empty_touches_js.py

@@ -0,0 +1,51 @@
+"""Pin canvasCoords (static/js/editor/canvas-coords.js) against an empty
+touch list. Driven through `node --input-type=module` (same approach as
+tests/test_markdown_table_row_js.py); skips when `node` is missing.
+
+Regression: a touch event whose `touches` list is present but EMPTY (a
+real mobile race — the finger is already lifted when the handler runs)
+made `e.touches[0].clientX` throw \"Cannot read properties of undefined\".
+The guard falls back to the event's own clientX/clientY in that case.
+"""
+import json
+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+
+_REPO = Path(__file__).resolve().parent.parent
+_MOD = _REPO / "static" / "js" / "editor" / "canvas-coords.js"
+_HAS_NODE = shutil.which("node") is not None
+
+_CANVAS = "{width:800,height:600,getBoundingClientRect:()=>({width:400,height:300,left:100,top:50})}"
+
+
+def _coords(event_js):
+    js = f"""
+    import {{ canvasCoords }} from '{_MOD.as_posix()}';
+    const canvas = {_CANVAS};
+    console.log(JSON.stringify(canvasCoords({event_js}, canvas)));
+    """
+    proc = subprocess.run(
+        ["node", "--input-type=module"],
+        input=js, capture_output=True, text=True, cwd=str(_REPO), timeout=30,
+    )
+    assert proc.returncode == 0, proc.stderr
+    return json.loads(proc.stdout.strip())
+
+
+@pytest.mark.skipif(not _HAS_NODE, reason="node binary not on PATH")
+def test_empty_touch_list_falls_back_to_client_xy():
+    # scaleX = 800/400 = 2; (200-100)*2 = 200, (100-50)*2 = 100
+    assert _coords("{touches:[],clientX:200,clientY:100}") == {"x": 200, "y": 100}
+
+
+@pytest.mark.skipif(not _HAS_NODE, reason="node binary not on PATH")
+def test_mouse_event_unaffected():
+    assert _coords("{clientX:200,clientY:100}") == {"x": 200, "y": 100}
+
+
+@pytest.mark.skipif(not _HAS_NODE, reason="node binary not on PATH")
+def test_touch_with_finger_still_used():
+    assert _coords("{touches:[{clientX:200,clientY:100}]}") == {"x": 200, "y": 100}