[panel plugin] Make stable option also take effect when a panel is removed

Issue codemirror#4485

Author: marijnh

addon/display/panel.js

@@ -57,6 +57,8 @@
     this.cleared = true;
     var info = this.cm.state.panels;
     this.cm._setSize(null, info.heightLeft += this.height);
+    if (this.options.stable && isAtTop(this.cm, this.node))
+      this.cm.scrollTo(null, this.cm.getScrollInfo().top - this.height)
     info.wrapper.removeChild(this.node);
     if (--info.panels == 0) removePanels(this.cm);
   };