Sorry about that. This is quite annoying and, unfortunately, quite hard to fix for everyone
Long story short: the lack of a global standard for CBR/CBZ file, plus the way iOS/MacOS associate apps with file extensions, makes it very hard for us to ensure that Panels appears in the list of compatible applications.
For each file format, iOS and MacOS use the file extension and something called UTI (Uniform Type Identifier), a “unique” identifier that the owner (creator) of the file format defines.
Given that CBR/CBZ are not standardized and there’s no official owner or organization defining the standard, every app defines their own UTI for these formats. iOS only allows 1 registered UTI per file extension. So, if you have more than one app that supports one of these formats, it might be replacing Panels in the registered apps list. Also, if you install one of these apps and then delete them, the UTI + extension registry breaks, unless another app is also registered to open those UTI. This means that users, can easily break their UTI registry just by installing a new app defining a new UTI.
File extensions that have an owner (like PDF) have a publicly standardized UTI that everyone uses. That’s why it less likely to see this bug happening with pdf files than CBR/CBZ.
Every time you open Panels, we try to find all the UTI registered on your device that Panels doesn’t support. And we add them to our supported list in the following releases. That minimises the chances of happening. We currently support 17 different UTI for CBR and another 17 for CBZ.
Hopefully, this issue should be fixed in upcoming releases. But unfortunately, it will probably resurface in the future. Because it’s not a bug in our app. It’s just that the standard (or lack of it) is completely broken.
Obviously, this is not unique to Panels. The folk that created Chunky Comic Reader wrote a great blog post about it.