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Instead of a blank field, make 'Assigned To:' a select box with list of all current user IDs (assuming Bugzilla user has rights to change bug information). This would prevent bad e-mail addresses from being entered in this field. (Note: I am working on this. If I get it working, I'll send the code.)
I assume you are talking about the "Assign bug to [ ]" box in the Change state or resolution box. That would normally be a good idea and would not be hard at all to implement, it is just that our Bugzilla currently has almost 25000 members and would not fit very well into a drop down box. I could sort the list by particular groups that the people belong to such as only display people who are members of 'qa' or 'devel' groups but you would still need to have the regular text box available for addition of people who are not in those. This would add more to the already large amount of clutter on the bug form. I would be glad to look at whatever you come up with.
The database I'm using will be smaller by a few orders of magnitude, but I understand the problem of making a 25,000 element scroll box. Perhaps some sort of secondary window would be better, allowing a search feature by name (kind of like the dialog for editmembers.cgi).
Bugzilla 2.17 allows email wilcards and email completion (enabled by a "usermatchmode" configuration parameter). When both are enabled, one can put "*@redhat.com" or "dkl" into any email field (CC, Assigned To, etc). When there is only one match, Bugzilla will just use it and when there are several matches (up to "maxusermatches" matches) Bugzilla will give a drop-down box with all of them. P.S. David, do you think this would be worth enabling in this Bugzilla?
Okay I have enabled this new feature and it seems to work real well, *sigh* one more thing I need to document. This definitely keeps the drop down list more managable since it doesnt limit it to only redhat.com addresses like some implementations. Please reopen if this is not satisfactory to the reporter of this bug.