1. Try selecting the whole line.
2. Copy-paste to somewhere else.

Expected: No selection for "noselect" span element
Actual: It's selected but omitted in clipboard, causing confusion

This also causes a web-visible behavior difference: getSelection().rangeCount becomes 2 without contenteditable but 1 with contenteditable.

I seem to recall it's intentional that user-select: none is ignored inside a contenteditable node - otherwise you wouldn't be able to edit it. Omitting it when it's copied to the clipboard seems like a bug though. I think the serialization/clipboard code has its own handling of user-select: none which we should probably nuke and just use the Ranges instead. (It was a long time ago that I hacked on Selection though, so I could be wrong.)

(In reply to Mats Palmgren (:mats) from comment #1)

I seem to recall it's intentional that user-select: none is ignored inside a contenteditable node - otherwise you wouldn't be able to edit it. Omitting it when it's copied to the clipboard seems like a bug though. I think the serialization/clipboard code has its own handling of user-select: none which we should probably nuke and just use the Ranges instead. (It was a long time ago that I hacked on Selection though, so I could be wrong.)

Yes, the serialization-code skips content with unselectable frames.

Given what Mats wrote above, it seems nsIFrame::IsSelectable should return true when the frame corresponds to a node which is contenteditable.