fix: Load preview URL for download-disabled shares

[susnux]

• ⚠️ chained on fix: Hide download action when file does not provide download permissions #2482
• ⚠️ Needs server changes: fix: Adjust preview for view-only shares server#47831

This was possible on Nextcloud 30 and previous due to a "bug": The download permission was simply not rejected for public shares, just a hide flag was set on the public share.

Now the permissions are correctly set, so loading a preview is not possible. The work-around is to allow previews when the correct header is set.

[skjnldsv]

Still valid @susnux ?

[skjnldsv]

Thanks for this, and sorry it sat so long. Both things it was chained on landed back in 2024 (#2482, and nextcloud/server#47831), and then nobody picked it back up.

Closing it here because this repository is frozen: the viewer now lives in nextcloud-libraries/nextcloud-viewer as @nextcloud/viewer, and master no longer takes changes. Only the stable branches are still updated until they go EOL.

Your fix is in the library as nextcloud-libraries/nextcloud-viewer#51, with a Co-authored-by for you. I wrote it fresh rather than cherry-picking, so it is worth saying what I changed and why:

• getPreviewIfAny stays synchronous. Making it async pushes both Images.vue and Videos.vue out of a computed and into watcher-and-ref shapes, and the library already refetches by hand when the element fails to load (for E2EE files). That retry now picks what to fetch based on canDownload, so the change is a few lines and an unrestricted file still loads straight from the URL with no extra request.
• Object URL rather than a FileReader data URL, so the image is not held as base64 in the DOM. Promise.withResolvers then isn't needed, and neither is the tsconfig bump.
• The refactors in your PR (PreviewUrl.js, models.ts, numberUtil.ts, BasicFileInfo) don't apply: that layout was reworked during the extraction.

The x-nc-preview mechanism itself is exactly as you had it.

One thing your PR made me notice: the existing fallback never revoked the object URLs it created, so paging through a folder of E2EE files held all of them until the viewer closed. That is fixed alongside.