When it comes to actions (by which I mean commands) in conventional UI, all desktop platforms follow the well established convention: keyboard acts on press, mo

When it comes to actions (by which I mean commands) in conventional UI, all desktop platforms follow the well established convention: keyboard acts on press, mouse acts on release. This doesn’t mean that there aren’t controls that react on mouse down—focusing an input field, for example, is a mouse-down thing, and you can immediately start things like text selection thereby; and draggable things like scrollbars are also naturally exempt from this. But basically everything else, certainly things that are for taking actions, should act on release, if you’re activating them with a pointing device. From time to time I encounter web systems that get this wrong, using keyup or keypress instead of keydown, or using mousedown instead of click¹. Most people won’t notice, I expect, but I suspect I pretty much always immediately notice and find them infuriating, because they break the universal convention—things that you’re used to being able to do, don’t work as you expect. Perhaps my favourite example of this was some system which would close some form when you released Escape—so when I had something else open above the browser at the OS level (or maybe it was even just a <select> dropdown! Actually that sounds right; I know I triggered this too often) and closed it by pressing Escape, the web page underneath then received the keyup, and so the Escape press closed two things in one go, which was never what I wanted. If you can start executing an action on mousedown and only commit it functionally and visually once it turns into a click, go ahead. But otherwise, please don’t act on press, because it’s wrong. The on-screen keyboard he mentions… well, I’m actually a bit puzzled by that, because it should completely obviously map press to keyboard press, and release to keyboard release, and anything else would be wrong, so how was there ever an argument, and in fact how else could you implement it? I genuinely don’t see any alternative, if you’re working on an OS that has the concept of keyboards already. Ultimately, I don’t think it supports his point at all, because he was implementing a keyboard. —⁂— ¹ Note that even monitoring mousedown + mouseup may sometimes be wrong, due to nuances that can cancel the click mid-press; things being draggable is the obvious example, but I think some platforms let you hit Escape on a keyboard, or maybe have other multi-pointer quirks.