Fix bug in selection model

Doing shift-left-arrow a few times and then shift-up a few times,
you'd get weird jumping selections as you went up.

Author: marijnh

lib/codemirror.js

@@ -569,13 +569,10 @@ var CodeMirror = (function() {
       // so that you can, for example, press shift-up at the start of
       // your selection and have the right thing happen.
       if (rs) {
-        from = sr.start == rs.anchor ? to : from;
-        to = shiftSelecting ? sel.to : sr.start == rs.anchor ? from : to;
-        if (!posLess(from, to)) {
-          reducedSelection = null;
-          sel.inverted = false;
-          var tmp = from; from = to; to = tmp;
-        }
+        var head = sr.start == rs.anchor ? to : from;
+        var tail = shiftSelecting ? sel.to : sr.start == rs.anchor ? from : to;
+        if (sel.inverted = posLess(head, tail)) { from = head; to = tail; }
+        else { reducedSelection = null; from = tail; to = head; }
       }
 
       // In some cases (cursor on same line as before), we don't have
@@ -854,10 +851,9 @@ var CodeMirror = (function() {
       if (posEq(sel.from, from) && posEq(sel.to, to)) return;
       if (posLess(to, from)) {var tmp = to; to = from; from = tmp;}
 
-      var startEq = posEq(sel.to, to), endEq = posEq(sel.from, from);
       if (posEq(from, to)) sel.inverted = false;
-      else if (startEq && !endEq) sel.inverted = true;
-      else if (endEq && !startEq) sel.inverted = false;
+      else if (posEq(from, sel.to)) sel.inverted = false;
+      else if (posEq(to, sel.from)) sel.inverted = true;
 
       // Some ugly logic used to only mark the lines that actually did
       // see a change in selection as changed, rather than the whole