step 3: add widget shell sizing and frame pump

src/dpg_map/interaction.py

@@ -1 +1,36 @@
 """Map interaction state and handlers."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+from .sizing import effective_draw_size
+from .state import MapState
+
+
+@dataclass(frozen=True, slots=True)
+class HitRect:
+    """Screen-space rectangle used for map interaction tests."""
+
+    x: float
+    y: float
+    width: float
+    height: float
+
+    @property
+    def right(self) -> float:
+        return self.x + self.width
+
+    @property
+    def bottom(self) -> float:
+        return self.y + self.height
+
+    def contains(self, x: float, y: float) -> bool:
+        return self.x <= x <= self.right and self.y <= y <= self.bottom
+
+
+def calculate_hit_rect(state: MapState, drawlist_pos: tuple[float, float]) -> HitRect:
+    """Return the map interaction rectangle for a drawlist position."""
+
+    width, height = effective_draw_size(state)
+    return HitRect(float(drawlist_pos[0]), float(drawlist_pos[1]), float(width), float(height))