Avoid premature trimming in RegionIntersection (#2471)

avoid premature trimming

worldedit-core/src/main/java/com/sk89q/worldedit/regions/RegionIntersection.java

@@ -164,15 +164,24 @@ public class RegionIntersection extends AbstractRegion {
         int bz = chunk.getZ() << 4;
         int tx = bx + 15;
         int tz = bz + 15;
+        List<Region> intersecting = new ArrayList<>(2);
         for (Region region : regions) {
             BlockVector3 regMin = region.getMinimumPoint();
             BlockVector3 regMax = region.getMaximumPoint();
             if (tx >= regMin.getX() && bx <= regMax.getX() && tz >= regMin.getZ() && bz <= regMax.getZ()) {
-                return region.processSet(chunk, get, set);
+                intersecting.add(region);
             }
         }
+        if (intersecting.isEmpty()) {
             return null;
         }
+        if (intersecting.size() == 1) {
+            return intersecting.get(0).processSet(chunk, get, set);
+        }
+        // if multiple regions intersect with this chunk, we must be more careful, otherwise one region might trim content of
+        // another region
+        return super.processSet(chunk, get, set);
+    }
 
     @Override
     public Future<?> postProcessSet(final IChunk chunk, final IChunkGet get, final IChunkSet set) {