Undo not working correctly

[k15a]

When switching the tab and undoing you get the code of the previous tab.

[jamesopti]

This is caused by CodeEditor's .setModeAndLinter being asynchronous.

If modes.set's promise is returned, then the clearHistory can be done after that.

this.setModeAndLinter().then(() => {
  this.codemirror.getDoc().clearHistory()
  this.focusLastCursorPosition()
})

I'll try to get a PR up for this.