I can't replicate it in Win2K SP4 with KM 1.5.4, when you said:
(that should open K-Meleon's logon chooser for that webpapge)
You are referencing to you are using "remember password" option to logon in this site?
This brings the list of stored user's logons to the site.
I was going to take screenshots and replicate it myself afterI read your reply, but it's hard to replicate for me now. I happened because my ISP had speed problems that day (I found it out later), now it is usual speed and before I can start dragging anything from another tab, the "stored password/user to choose from" already pops up and I can't drag because the focus already switches to that pop up before I can drag anything.
But I have an idea. I'll try to replicate it by tweaking connection speed for my computer on the router box, or (if that won't do) I'll try with using dial-up connection, this should simulate slow connection well, because as I said earlier this happened when my ISP had some major problems that day in my area and my connection speed was very slow for short period of time (which is probably why the stored passwords pop-up appeared with delay). If you have modem maybe you can try it too?
Open 1 tab to a site where you login to, so the stored user/password will pop up when that page is loaded, hopefully few seconds later (i.e. gmail or something like that) and in the meantime open some different page in another tab and start dragiing i.e. the URL to the desktop, but don't drop it yet, just hold it with your mouse and wait for the "chosing user/password" pop up from the first tab to appear. This should do: what will happen is once the pop-up appears, you won't be able to drop the dragged URL to the desktop, the mouse cursor will indicate that you are dragging/dropping but when you release mouse button nothing will happen. And in the same time you won't be able to choose your user from the stored passwords pop-up window either. You will have to open Task Manager and kill frozen K-Meleon.
I'll try to take video if I'll succeed replicating this.