Until sometime ago I was able to copy the text of a collapsible section header such as this:
https://en.wikipedia.org/wiki/WP:Sandbox?oldid=592466923
It seems this broke when rMWe1159798bd1543f819b45d23e2f4632f85f017b2 was merged.
Tested both on Google Chrome and Firefox.
See Also: T36445: user-select:none for editsection buttons and other buttons