Thebe clashes with hide-cell tag in Jupyter-book

[fortierq]

Describe the bug

Thebe prevents cells with hide-cell tag from opening in Jupyter-book. "Click to show" does nothing. This is because the ID of the div is overwritten by shpinx-thebe, and the "click to show" button uses the original ID to toggle visibility.

To Reproduce

Steps to reproduce the behavior:

1. Define a code cell for Jupyter-book:

```{code-cell} ipython3
:tags: [hide-cell]
(1 + 5**0.5)/2
```

2. Launch Thebe inside Jupyter-book
3. "Click to show" does not show hidden cell.

Expected behavior

"Click to show" should show the hidden cell.

Environment

• Python Version: 3.8.5
• Package versions or output of jupyter-book --version: 0.11.1
• Operating System: Ubuntu

Additional context

The following line of code change the id of an hidden cell, preventing to toggle its hidden status:

sphinx-thebe/sphinx_thebe/_static/sphinx-thebe.js

Lines 62 to 65 in 53c098b

	const codeCells = document.querySelectorAll(thebe_selector)
	codeCells.forEach((codeCell, index) => {
	const codeCellId = index => `codecell${index}`;
	codeCell.id = codeCellId(index);

Why do we need to change this codeCell.id? Can we modify only the id of non-hidden cells?

I would happily try to submit a PR if needed :)

PS: in https://jupyterbook.org/interactive/thebe.html, an example is provided with a hide-input cell. This works because the corresponding HTML element has class cell-input and not class cell (like hide-cell tag), and thus its id is not modified.

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[choldgraf]

@fortierq thanks for opening this one up - I think that your diagnosis is correct that we are overwriting and ID. I think that we could try:

If a cell already has an ID, then we don't assign it a new one. If we can confirm that the behavior still works the same way, then maybe we are fine to stop there.

Want to give that a shot?

[fortierq]

Thanks @choldgraf! I'm afraid I'm too busy at the moment to work on this at the moment. Maybe later.