Fix a bug in preserving fixed state.

d3.v2.js

@@ -4177,17 +4177,17 @@
     return d3.rebind(force, event, "on");
   };
   function d3_layout_forceDragstart(d) {
-    d.fixed |= 1;
-  }
-  function d3_layout_forceDragend(d) {
-    d.fixed &= 2;
-  }
-  function d3_layout_forceMouseover(d) {
     d.fixed |= 2;
   }
-  function d3_layout_forceMouseout(d) {
+  function d3_layout_forceDragend(d) {
     d.fixed &= 1;
   }
+  function d3_layout_forceMouseover(d) {
+    d.fixed |= 4;
+  }
+  function d3_layout_forceMouseout(d) {
+    d.fixed &= 3;
+  }
   function d3_layout_forceAccumulate(quad, alpha, charges) {
     var cx = 0, cy = 0;
     quad.charge = 0;

src/layout/force.js

@@ -292,20 +292,26 @@ d3.layout.force = function() {
   return d3.rebind(force, event, "on");
 };
 
+// The fixed property has three bits:
+// Bit 1 can be set externally (e.g., d.fixed = true) and show persist.
+// Bit 2 stores the dragging state, from mousedown to mouseup.
+// Bit 3 stores the hover state, from mouseover to mouseout.
+// Dragend is a special case: it also clears the hover state.
+
 function d3_layout_forceDragstart(d) {
-  d.fixed |= 1;
+  d.fixed |= 2; // set bit 2
 }
 
 function d3_layout_forceDragend(d) {
-  d.fixed &= 2;
+  d.fixed &= 1; // unset bits 2 and 3
 }
 
 function d3_layout_forceMouseover(d) {
-  d.fixed |= 2;
+  d.fixed |= 4; // set bit 3
 }
 
 function d3_layout_forceMouseout(d) {
-  d.fixed &= 1;
+  d.fixed &= 3; // unset bit 3
 }
 
 function d3_layout_forceAccumulate(quad, alpha, charges) {