I guess I do not see any bug here. "or be told that no preview is available." is basically what already happens.
https://nv.wikipedia.org/api/rest_v1/page/summary/Kéyahígíí nidineesdzaʼ has empty extract and extract_html.

In T309760#7975982, @Aklapper wrote:

I guess I do not see any bug here. "or be told that no preview is available." is basically what already happens.
https://nv.wikipedia.org/api/rest_v1/page/summary/Kéyahígíí nidineesdzaʼ has empty extract and extract_html.

If "There was an issue displaying this preview" can only mean no preview is available it should say that. But it sounds like a generic fallback error.

I suggested "be told that no preview is available" as that's common wording to tell the user there is no preview and this was the expected result. When you say "there was an issue", frowny face included, a preview was expected to be available, it should have been there, it COULD have been there, but unfortunately, something crashed or didn't respond or didn't parse or your browser can't handle it or whatever and now, unfortunately, there's no preview.

I think we can rephrase this issue. In the particular example given the REST endpoint does return a valid response. It just happens that the page contains no text and the text snippet reported by the endpoint is an empty string because of this. But there is a thumbnail. This could be shown as a normal page preview popup – just with no text. Instead the code throws the thumbnail away and continues as if the REST query failed.

I think we can easily change this to display the thumbnail and the page title again, just so that the text is not empty.

Change 822443 had a related patch set uploaded (by Thiemo Kreuz (WMDE); author: Thiemo Kreuz (WMDE)):

[mediawiki/extensions/Popups@master] Fall back to title and thumbnail when page doesn't have text

https://gerrit.wikimedia.org/r/822443

@ovasileva @alexhollender there's an open patchset to change the behaviour of page previews when the preview text can't be received to show title/description. While I agree the existing error is a little generic and makes the user feel like they've done something wrong I'mm not sure showing the preview without the summary text makes sense.

Could we at least provide a link to "Troubleshoot" perhaps pointing to a generic FAQ? In this case https://nv.wikipedia.org/wiki/K%C3%A9yah%C3%ADg%C3%AD%C3%AD_nidineesdza%CA%BC has no text in it which an editor could easily fix.

I'm not sure there is anything to "troubleshoot" when a gallery page is intentionally designed to show one or more images, but no text. I'm afraid such a message can trick users into making edits that should not be made.

@Jdlrobson, @thiemowmde - to make sure I'm following - this proposal is for cases where a preview might be available but does not exist. I would argue this is close enough to the case for it not to warrant a special case. However, I think a change in copy can also be acceptable - I'd say that would be preferable to introducing a new, thumbnail-only type of preview.

a preview might be available but does not exist.

I'm afraid I don't understand this.

introducing a new, thumbnail-only type of preview.

I would argue that there is nothing new here. It's the same popup – either horizontal or vertical depending on the image – and it shows exactly what the user will find on the target page. Maybe a screenshot helps?

Creating the impression there would be anything wrong with a target page like this is not a good idea, for all I know and care about. We cross a line between "reader space" and "editor space" here. A message that talks about an error is likely to trick readers that try to be helpful into conflicts with the authors that maintain these pages.

My WMDE-TechWish team just removed a series of warnings and notices from the VisualEditor template dialog for this reason. For example, we had a notice like "the value you entered is not one of the known suggested values". But no matter how defensive we wrote this, it made users think there is an error and they started making things worse by trying to "fix" them. Same here. Even a placeholder like "there is no text on the page" can have this effect. And it's not needed in the first place. I can see there is no text.

Let's be clear this is an edge case. What we are talking about here is an article with just images on it and no text:
https://nv.wikipedia.org/wiki/K%C3%A9yah%C3%ADg%C3%AD%C3%AD_nidineesdza%CA%BC

Showing a preview with the title and image doesn't really add any value IMO. It just creates confusion (why am I not gettin a proper preview for this page? What has this image got to do with the title?"
We could put a placeholder as you suggest saying "there is no text on the page" but that's also going to be confusing. A page with no lead section will also generate no preview as will an article which begins with <p class="noexcerpt">Text</p>:

== Heading 1 ==
text

There are a ton of other edge cases which could generate no summary that I won't go into now.
Personally this task seems like a super low priority. I think the initial recommendation of improving the copy is fine here, but I don't think this bug warrants any big changes to page previews right now.

I'm not sure we talk about the same issue. "Users would worry that it was broken" is what happens right now with the misleading message, and what I try to fix with https://gerrit.wikimedia.org/r/822443. I don't understand how the current situation can become worse when we make the popup show exactly what can be found on the target page?

Per the web team's quarterly grooming, these tasks are being removed from the team's backlog.

Change #822443 abandoned by Thiemo Kreuz (WMDE):

[mediawiki/extensions/Popups@master] Fall back to title and thumbnail when page doesn't have text

https://gerrit.wikimedia.org/r/822443