Well we need to take over action=edit if that's how the user has set their preferences. Our hack to prevent multiple editors being shown in most cases prevents the VE client from always being able to pull the text given by the wikitext editor. So we're left with generating the list ourselves...

https://en.wikipedia.org/wiki/Category:All_stub_articles is the largest category on enwiki with just under 2 million articles (and it's not just as simple as articles, there's files and subcategories that you also have to account for with some categories). This means it uses the paging system, involving 'pagefrom' and 'pageuntil' parameters and only showing 200 items at a time.

And I imagine this type of issue applies to more than just category pages, because others such as file pages would have similar issues I expect?

In T139191#2460461, @AlexMonk-WMF wrote:

And I imagine this type of issue applies to more than just category pages, because others such as file pages would have similar issues I expect?

Maybe. Though files without a description page are pretty rare. And even then, while pages do have a default "view" (showing upload links and a "File usage" what-links-here section), that view is not shown when visiting redlink=1 nor action=edit. Whereas category pages do show that (which makes sense, since redlinks to categories are much more exposed, and are often reached before the category description is made).

With patch https://gerrit.wikimedia.org/r/441505, it will at least be possible to close VE/NWE after it opens (by pressing Escape), which will redirect to the (usually inaccessible) view URL. This is still not great behavior though.

Note also that when VE/NWE is opened from the view URL (i.e. https://www.mediawiki.org/wiki/Category:MediaWiki_Usage_Report_2015?veaction=edit / veaction=editsource), it actually will display the list of pages under the editor; but the links in the list are not clickable, so that is somewhat useless in practice. This special handling also causes T194068, so I would be inclined to remove it.

Change 441509 had a related patch set uploaded (by Bartosz Dziewoński; owner: Bartosz Dziewoński):
[mediawiki/extensions/VisualEditor@master] Remove special handling for category and file pages

https://gerrit.wikimedia.org/r/441509

In my opinion, the best solution for this task would be to make action=edit&redlink=1 URLs redirect to the view URL for pages in Category: and File: namespaces. I think that would definitely be a better behavior for VE, and probably also for the old/built-in wikitext editor.

Category pages, even when they don't "exist", still display content. File pages almost never should be created manually, but rather by uploading a file.

Note that action=edit&redlink=1 URLs already redirect to the view URL in two cases: if the page actually exists, or if the user doesn't have permission to create it (try a non-existent main namespace page while logged out on enwiki).

(Patch is only tangenially related)

Maybe this should be solved before T84936: Release VisualEditor-MediaWiki as "1.0"?