Background
IIIF (International Image Interoperability Framework) icons (specifically, IIIF manifest URLs) support loading via drag & drop in many IIIF-compatible viewers.
However, I received feedback that the IIIF icons displayed by the following module did not support drag & drop.
https://github.com/omeka-j/Omeka-S-module-IiifViewers
I investigated the cause by referring to the following page, among others.
https://zimeon.github.io/iiif-dragndrop/
As a result, I found that the following HTML markup is required.
Not conforming to the above pattern was the cause of the drag & drop issue in the aforementioned module.
This article describes the solution.
Solution
Specifically, consider the following manifest URL as an example.
Manifest URL example: https://omekas.aws.ldas.jp/sandbox/iiif/2/2/manifest
Previously, the href attribute of the a tag only contained the IIIF manifest URL, as follows.
This needed to be modified as follows.
The behavior when clicking the icon remains the same (downloading the manifest), but the latter version supports drag & drop to other IIIF-compatible viewers.
While writing the manifest URL twice in the href attribute is somewhat redundant, the same approach is used in Universal Viewer and IIIF Curation Viewer.
I also learned that since the ?manifest=manifest_URI part is typically used for loading, the default_target part before it often has no effect.
Therefore, I confirmed that URLs and icons for Universal Viewer and IIIF Curation Viewer, which often have URLs like the following, can also be dragged and dropped into other IIIF viewers. (I’m a bit embarrassed to admit that I previously thought the default_target part needed to be the IIIF manifest URL.)
Summary
I hope this helps when placing IIIF icons (IIIF manifest URLs) on your site to support drag & drop functionality.