Multiple files in Web IDE

I have a quick prototype of this at: https://gfwilliams.github.io/EspruinoWebIDE/

You can now have multiple tabs. For each file you open you get a new tab - and for each tab, the way it's saved is remembered (as well as - finally - the filename!).

Right now it:

• doesn't understand about blockly as a type of Tab (the graphical language)
  
• re-uses the same editor, so Undo isn't remembered if you flip between tabs
  
• doesn't have a 'new tab' button
  
  

What do you think about this? Is it useful? Would it help? Anything glaring that's wrong with the way it works?