There is a cancel button while the image is loading (or if there isn't one, that's a regression) but not while MediaViewer itself (the ResourceLoader module) is loading. There is a small head-loaded module (mmv.head) which sets click handlers and sets up a black overlay on click and loads the full script, which is the one that sets up the cancel button and the rest of the UI. Not sure how much sense that setup makes now with RL being fully async (and a lot faster than it used to be).

Maybe the solution is to preload the module with the progress bar and the cancel button already on page load and not only on clicking an image.

Mass-removing the Multimedia tag from MediaViewer tasks, as this is now being worked on by the Reading department, not Editing's Multimedia team.