Can't update `children` trait after widget is displayed.

[bollwyvl]

Akin to #1059, here's another fun gist with binder that demonstrates some issues observed while try to upgrade to 6.19.x.

Basically:

• Creating a HBox with children displays them immediately
• Creating an HBox and then setting children doesn't show anything at all
  • well, the .widget-hbox gets created, but its empty, and never updates

Again, I'll do a bit more debugging.

[bollwyvl]

I've added kernelspy to the gist.

6.17	6.19

[bollwyvl]

Here's the expanded comm_msg missing in in 6.19: sure enough, it's the one that sets the children.

[bollwyvl]

In notify_change, ipywidgets checks if the comm.kernel is None, this appears to always be the case, even for previously-established comms.

https://github.com/jupyter-widgets/ipywidgets/blob/f89c5d983d534fe3bbf8440f085e228ee89d681c/python/ipywidgets/ipywidgets/widgets/widget.py#L689

[maartenbreddels]

Ok, this was the bug I was hunting, didn't expect this. Thanks again 🙏

[bollwyvl]

This was a good exercise: i guess my lesson learned was that going right to the message trace its where it's at for these things.

kernelspy is fine, but not very fine-grained, and running the full lab UI requires so many downstreams that it might be limiting.

Perhaps something lower level that allowed for capturing full traces as JSON lines would be a good feature... building it into the kernel itself would be... something, but probably makes more sense to do it out-of-band so that you can run it against other kernels, akin to jupyter_kernel_test, but maybe something that worked with an existing forensic tool, e.g. kernelshark for wireshark.