Yes you're right, seen that too doing testing before, quite annoying when happens, and also for me happens very intermittently. I just tried again after reading your post and remembering issue, but typical now cannot reproduce whatsoever, tried 15-20 times or so, switching between two magnets to trigger the dialog.
I am not good at coding, and for sure not GUI stuff, so honestly cannot help - I will report if can find a way to reproduce fully but e.g. as seen now, then sometimes quite impossible
When testing that magnet stuff previously we both tested, then indeed happened several times for me, just not now.
Personally I found that the modal dialog, not sure if correct terminology, but anyway it's hidden behind another, when happens, or atleast i'm guessing, so a timing issue maybe, but unsure frankly - anyway if pressing escape on keyboard then removes that hidden in back dialog and you can press 'url' again and continue - but your workaround is just as quick, so thanks for posting altenative workaround.
hmm, maybe should make ticket anyway, as is an issue for sure when we both see it, and even though a dev maybe cannot reproduce, then they can probably check the code for any timing or whatever issue is the problem.
I'll do it later, thanks for posting/reporting again!
Edit: Sorry just saw your blog video, and that is another issue then what I saw. I'm thinking it's a KDE issue, as don't see that here i.e. that the window doesn't focus after adding magnet, only focuses the dialogs for adding torrent. I use a tiling window manager and here does swicth focus - but as you said doesn't happen alway, so maybe I haven't catched it, or noticed even, and so probably not KDE related sorry and probably is related nonetleless, so i'll add it to report later, thanks again.
Edit2: For my issue I'm guessing a slight twisted callLater or callback or how can be done so the addtorrentdialog component is having time to show() on screen before it's next add_button_url_clicked() gets displayed, but as said cannot reproduce now so hard test it. Anyway, this clipboard detection functionality was added into a release with deluge 2.0.4, so do you know if your issue is new, as in deluge 2.1.0 and not before, or you're unsure? Strange because code run only when window focused, so how can loose it again, but your video shows it nonetheless.