>No, that's actually ungrammatical, though it's widely used. "He or she" is correct, though in your sense, you can avoid the problem entirely and make the sentence better <g>: "When the user clicks the login button, the main screen appears"
>
>General rule of strong writing is remove as much as you can, but no more.
>
>Tamar

I agree that the re-write of the sentence is better, but... my understanding is that the grammatically correctness of the use of the singular they is debated (and I agree that I might have gotten the whole concept wrong). As I said, there is also a small, very very small chance that I misunderstand the whole issue :)

From http://en.wikipedia.org/wiki/Gender-neutral_language_in_English

Pronouns [edit]
See also: Gender-neutral pronoun
The use of masculine pronouns to refer to antecedents of mixed or indeterminate gender, while traditional, is a target of frequent criticism by proponents of gender-neutral language. Critics of the use of the generic "he" argue that this usage was invented and propagated by men, such as Thomas Wilson and Joshua Poole, whose explicit goal was the linguistic representation of men's superiority.[18] The use of the generic "he" was in fact enforced by an Act of Parliament[19] and, despite its putative inclusiveness, has been used to deny women's entry into professions and schools.[5]
Proposed alternatives to the generic "he" include "he or she" (or "she or he"), "s/he", or the use of "they" in the singular; each of these alternatives has met with objections. Some feel the use of the singular "they" sounds like a grammatical error, but according to some references, "they", "their", and "them" have long been grammatically acceptable as gender-neutral singular pronouns in English: