When I try to edit a category page, the the markup is visible for a brief moment as the editor loads, but after that it becomes invisible (though still selectable). A workaround is to temporarily disable syntax highlighting.
I have tested this in Firefox 52.6.0 and Chromium 62.0.3202.89. In both browsers I have the HTTPS Everywhere and Privacy Badger Extensions installed.
Confirmed. Suspect this is somehow due to differences in the views between the normal Category view, and the parsoid version (without category tree)
https://en.wikipedia.org/wiki/Category:Coordinates_on_Wikidata -> then click edit.
Works when you directly go to
https://en.wikipedia.org/wiki/Category:Coordinates_on_Wikidata?action=edit
Another workaround than disabling syntax highlighting, is just refreshing the page.
I could not confirm this workaround.
No, I have two edit tabs. This is the 2017 wikitext editor (a.k.a. "new wikitext mode").
Here's something interesting: The "refresh the page" workaround works if I have only one edit tab, but not when I have two.