I just think that it the apps that make it necessary shouldn’t do so: if Photoshop (or GIMP, or LibreOffice, or whatever) takes 30 seconds to open a file, the solution shouldn’t be to layer on yet another option and make me think whether I want to suffer that delay or reduced functionality, it should be to eliminate the delay.
Yet (as I’ve also said) I don’t think anybody came up with a plausible way to do so (even a purely technical one, disregarding economic incentives for app developers).
Quickly flipping between files with horizontal arrows (or something of a similar complexity) is actually the more interesting function here, I think, and maybe it does warrant a mode. Some picture viewers try to make the switch implicit (flip between files when picture fits on screen, pan horizontally when not), but my builtin mode tracker gets out of sync frequently, and I would absolutely hate to have the same error-prone controls in a PDF viewer, where I sometimes stare at a single file for hours and sometimes flip through a dozen in a minute. No idea how this should be done.
> Being able to check the date or bottom line on a received PDF invoice, without opening it.
I tried double-clicking on a 43-page PDF, and it took less than a second to open on my under-powered Acer (about ~$200, mfg 4 years ago), running stock Ubuntu 20.04. This seems really fast, as it was basically instantaneous from releasing the mouse. Obviously opening up a PSD would be slower, but then again I can always just zoom in on the thumbnail in that case (Ctrl+Scroll zooms in on icons in Linux).
This is making me think perhaps the real difference is that the default programs that open are too slow, eg if Photoshop is the default associated application for all images, or Adobe Acrobat for all PDFs, then I totally would see the need to have a previewer as well, since last time I used those they were realllllly slow.
I dunno, that said, it's all really not important. Then again, what is HN but for getting grumpy about unimportant things? :p
When I'm browsing a folder of my image files and can't remember which of dozens of files is the one I actually want, QuickLook lets me pop open a screen sized modal preview window, which is as good as having that file open in the app. If it's not the file I'm looking for, I hit the up or down arrows and the next or previous file's content is displayed in the already open preview popup. If some of those other image files were created by Illustrator, I've just saved myself an additional ~30 seconds, by not having to open that app too. And, once I find the image I want, I can open it in its associated app directly from a button in the top of the preview pane.
But, even if you're not a graphic designer, QuickLook has its uses for other types of document too. It can preview the content of source code files [with syntax highlighting] preview the rendered output of things like HTML, PDF, Markdown, AsciiDoc etc. You can even scroll through the pages of a multipage document.
This is another timesaver I use all the time. Being able to check the date or bottom line on a received PDF invoice, without opening it... being able to check whether I mentioned something in a letter I sent, without firing up a text editor or word processor... being able to see a rendered representation of what an HTML file contains... etc. etc.