[panel addon] Fix changed method of panels

Closes #4683

addon/display/panel.js

@@ -66,7 +66,7 @@
   Panel.prototype.changed = function(height) {
     var newHeight = height == null ? this.node.offsetHeight : height;
     var info = this.cm.state.panels;
-    this.cm._setSize(null, info.height += (newHeight - this.height));
+    this.cm._setSize(null, info.heightLeft -= (newHeight - this.height));
     this.height = newHeight;
   };