make selection box sub-tile aware

src/test/BuildingTest.cpp

@@ -11,6 +11,7 @@
 #include <vector>
 
 #include <QPoint>
+#include <QRectF>
 
 #include "BeltSystem.h"
 #include "Building.h"
@@ -135,6 +136,26 @@ TEST_CASE("BuildingSystem: place miner occupies expected body tiles", "[building
     REQUIRE_FALSE(isTileOccupied(f.state, QPoint(1, 1)));
 }
 
+TEST_CASE("buildingsInBox covers a body cell the box only reaches into", "[building]")
+{
+    PlacementFixture f;
+
+    const BuildingId id = f.bs.place(f.state, BuildingType::Miner, QPoint(0, 0),
+                                     Rotation::East, 0).value();
+
+    // Body at (0,0),(1,0),(0,1). The box is unsnapped and lies wholly within cell
+    // (1,0) without filling it, which is enough: a building is covered when the box
+    // overlaps any of its body cells (REQ-UI-MULTI-SELECT, Coverage).
+    const std::vector<BuildingId> grazed =
+        buildingsInBox(f.state, QRectF(1.6, 0.4, 0.2, 0.2));
+    REQUIRE(grazed.size() == 1);
+    REQUIRE(grazed.front() == id);
+
+    // (1,1) is the output-port tile, not a body cell, so a box inside it covers
+    // nothing even though it is surrounded by the miner's cells.
+    REQUIRE(buildingsInBox(f.state, QRectF(1.2, 1.2, 0.5, 0.5)).empty());
+}
+
 // -- World-bounds rejection (REQ-BLD-PLACE-VALID) ---------------------------
 
 TEST_CASE("BuildingSystem: place rejects a building above the world (y < 0)", "[building]")