So the use of a plural implies it will do more than one. If it will only do the single one selected it should say “Rebuild Thumbnail” singular. If you then select more than it should have the plurals. As it is, it implies to me that it would rebuild all thumbnails.
Were it me I would change that behavior, that is of course, up to you. But I would have a “Rebuild All Thumbnails” menu item as well. This would be both a quality of life feature, and help subtly educate what the
I think the app telling you these images don’t have embedded thumbnails (which can happen), or they aren’t big enough for a good user experience, with an option to never see the dialog again is a good thing. Settings could then have the option to set what that threshold is. It is possible I might want them bigger than “normal” for a given value of normal, depending on the subject matter.
I also think you should have at least the option to do these operations (and rotate) in the background and not make the user wait. There are 4+ cores on the systems you support (7+ for the ARM macs), and Grand Central Dispatch is a wonderful thing… when you can use it. And I think it is appropriate to say the behavior needs to be different on different platforms. However reducing the behavior to the Lowest Common Denominator can introduce UX issues.
I want to make it clear, you have made something wonderful, I think this program is amazing. The tone I struck isn’t something I am most proud of, nor was it directed directly at you, but to another contributor; regardless I apologize to both of you. That being said I don’t think I am being unreasonable in saying allow for background processing where it makes sense, scale the experience to the platform, expose the behavior more so the user is understanding what is happening. And if we differ on that, fair enough