Steps to reproduce the bug: 1) Go to a password protected website which needs to enter username/password with a dialog window. 2) enter a wrong username/password. 3) you'll be prompted again to insert the right username/password but the username field is locked. 4) enter a password. 5) the dialog will be re-opened, but now the username field is available. For example try: https://webmail.unipd.it/ (I'm using r894391)
reporter of bug #178415 has the same problem on twitter plasmoid.
*** Bug 178415 has been marked as a duplicate of this bug. ***
Bug 170739 seems to be related to this.
*** Bug 170739 has been marked as a duplicate of this bug. ***
*** Bug 175489 has been marked as a duplicate of this bug. ***
I hit this bug in the release version of KDE 4.2 (Gentoo). Example of a site that shows the bug even on first access of the login page: https://geco.reply.it/gecoprod/
*** Bug 189085 has been marked as a duplicate of this bug. ***
Seems fixed in KDE 4.3. (And I could reproduce this in 4.2, too)