I think I found an issue in the JS FWT. Below is code you can run via the JS FWT Demo.

In the SWT implementation, the red background goes away when you switch tabs. This is the expected behavior. However, in the JS implementation, the red background persists.

// Save as Test.fan and access via http://localhost:8080/Test.fan 
// when running the JS Demo code

using gfx
using fwt

class Test
{
  Void main()
  {
    Window
    {
      TabPane
      {
        Tab { text = "Red"; RedBox(), },
        Tab { text = "Other Stuff"; Label { text = "Other stuff!" }, },
      },
    }.open

  }
}

class RedBox : Canvas
{
  override Void onPaint(Graphics g)
  { 
    w := size.w
    h := size.h
    g.brush = Gradient("0% 0%, 100% 100%, #f00, #f00")
    g.fillRect(0, 0, w, h)
  }  
}

Promoted to ticket #1845 and assigned to andy

Probably a z-index order bug.