The code for onCopy is pretty terse:

My best guess is that the following is true in FF but also in other browsers:

if (
  hasEditableTarget(editor, event.target) &&
  !isEventHandled(event, attributes.onCopy)
) {

which then prevents the default event, and then calls setFragmentData, but if the selection is empty, setFragment data returns:

@dylans thanks for looking into it 😄. If I add an event listener breakpoint for the copy event firefox it doesn't get triggered as well though so it seems to me like it doesn't get triggered at all. I've been trying to reproduce this independently from slate without success though (just react 17 and some content editable code) so I might be missing something here.

As for the setFragment there, the selection isn't null in the case I was trying to describe above but rather collapsed, so it shouldn't halt there (I just noticed I didn't really mention that clearly in the steps). The case for which this causes problems is when the cursor is on a void node as then the void node is not copied.

I'm experiencing the same. What is interesting is that it works in the images example (https://www.slatejs.org/examples/images), but not when I try to do the same in my code.

If you only select a void block in Firefox, onCopy and onCut and onCopy events doesn't trig at all, as @bryanph says.

However if I press shift+arrowLeft and keep the single void selected, it will trig the events afterwards (though the editor slate selection object is still the same).

I suspect it is related to there not being a real DOM-range set and Firefox thinks nothing is selected and this will not fire the events.

I see we have some special Firefox code hacks here and there (el.focus()), could it be related? Maybe some special treatment for FF is missing?

I'm drafting a PR with a potential fix for this: #4775

@skogsmaskin Do you use react 17.x in your code? The examples in slate are run using 16.x I think. I don't quite remember 100% but I thought this only occurred in react 17 for me. If that's the case then there might be some extra code in react 17 that prevents the oncopy event from being triggered or something. In any case thanks for looking into this 😄

Also just debugging this is very inconsistent, I have no idea what's going on for example here I have the exact same codesandboxes and it works in one and not the other? 😕

Working https://codesandbox.io/s/slate-react-17-oncopy-bug-working-twndi?file=/index.js
Not working: https://codesandbox.io/s/slate-react-17-oncopy-bug-not-working-ye09p

(and trying a little later both work again 🤯)

Perhaps it's some kind of async problem?

From #4775 (comment)

It's a react bug, see facebook/react#23094