Yes, the usual copying of the code through ctrl+c or the context menu and pasting into a text editor.
You are right, there is no guaranteed solution. I now checked it in IE11 and this does not work.
Also I noticed another parameter GESHI_HEADER_PRE_TABLE, I do not know why I did not test it before, but in IE11, Firefox 72.0.2, Chrome 79 and Edge it works fine. Of course, it may not work in some older versions of browsers.
About specially “header” attribute about which I wrote. Instead, the plugin may allow the user to modify the $geshi object through a filter in functions.php where user can set type of code container.
