Clicking the large 800px thumbnail on a file description page leads to the source image being viewed as if it were a page in the browser. This kind of made sense as a quick hack for enlarged views when image resize was first introduced. But for files with mustRender(), the behaviour is undesirable, since the browser's behaviour when the user clicks on that link is typically surprising and unpredictable.
This came up on IRC recently when a user complained that clicking on a description page thumbnail which has been rotated using the new EXIF autorotation feature resulted in an incorrectly rotated enlarged image being displayed. Such images have mustRender() == true.
I suggest making thumbnail clicks for files with mustRender() == true do nothing, i.e. don't wrap that image with a link. Instead, provide a text link to the source file in the area below the thumbnail.
Ideally thumbnail clicks of images with mustRender() == false would provide some kind of sensible, interactive enlargement interface, rather than causing all navigation to disappear and instructing the browser to download and decompress potentially hundreds of megabytes of image data.
See Also: T67383: Generate optimised thumbnail even when dimensions match original