Revert part of 9ad3837

It breaks document tree rebalancing.

Closes codemirror#4032

Author: marijnh

lib/codemirror.js

@@ -7342,31 +7342,25 @@
     },
     // When a node has grown, check whether it should be split.
     maybeSpill: function() {
+      if (this.children.length <= 10) return;
       var me = this;
-      var children = me.children;
-      var numChildren = children.length;
-      if (numChildren <= 10) return;
-      // To avoid memory thrashing when the children array is huge (e.g. first view of a large file), it's never spliced.
-      // Instead, small slices are taken. They're taken in order because sequential memory accesses are fastest.
-      var parent = me.parent || me;
-      var numMoreBranches = Math.ceil((numChildren - 10) / 5);
-      var firstBranchNumChildren = numChildren - numMoreBranches * 5;
-      var firstBranch = new BranchChunk(children.slice(0, firstBranchNumChildren));
-      var branches = [firstBranch];
-      firstBranch.parent = parent;
-      for (var i = 0; i < numMoreBranches; i++) {
-        var branchStart = firstBranchNumChildren + i * 5;
-        var branch = new BranchChunk(children.slice(branchStart, branchStart + 5));
-        branches.push(branch);
-        branch.parent = parent;
-      }
-      if (parent === me) {
-        parent.children = branches;
-      } else {
-        var myIndex = indexOf(parent.children, me);
-        parent.children.splice(myIndex, 1, branches);
-      }
-      parent.maybeSpill();
+      do {
+        var spilled = me.children.splice(me.children.length - 5, 5);
+        var sibling = new BranchChunk(spilled);
+        if (!me.parent) { // Become the parent node
+          var copy = new BranchChunk(me.children);
+          copy.parent = me;
+          me.children = [copy, sibling];
+          me = copy;
+       } else {
+          me.size -= sibling.size;
+          me.height -= sibling.height;
+          var myIndex = indexOf(me.parent.children, me);
+          me.parent.children.splice(myIndex + 1, 0, sibling);
+        }
+        sibling.parent = me.parent;
+      } while (me.children.length > 10);
+      me.parent.maybeSpill();
     },
     iterN: function(at, n, op) {
       for (var i = 0; i < this.children.length; ++i) {