Make button un-selectable

[mgeier]

When you do not use the copy button to copy text, but you select text in the old-fashioned way to copy it, you might also inadvertently select the button itself. This especially happens when selecting multiple adjacent code blocks.

This is a minor aesthetic glitch, but the real problem is that the alt text (typically "Copy to clipboard") ends up polluting the clipboard!

[choldgraf]

I'm not super familiar with this CSS option but it seems like a good idea to me! Thanks for the patch

[mgeier]

I've just discovered this recently in the context of code blocks with line numbers, where it totally makes sense not to select the line numbers.

I've also added it in nbsphinx in spatialaudio/nbsphinx#439.

After that, it should be able to select code from multiple adjacent Jupyter code cells, without the prompts or copy icons messing anything up.

[choldgraf]

nice - that sounds quite useful!