In the Chromium prototype, and in the explainer, light dismiss happens on mousedown. I implemented and wrote it that way based on a quick survey of both native and web popup implementations - all seem to dismiss on mousedown. Additionally, using mousedown instead of mouseup allows someone to mousedown on the popup and drag off of the popup before mouseup, for example when selecting text. Because the mousedown was on the popup, no light dismiss happens. In a local prototype, it was fairly jarring when implemented on mouseup, when trying to select text on the popup: it would close inadvertently fairly often.

However, @scottaohara pointed out that this might be less accessible, for example for someone with motor disabilities who accidentally clicks outside a popup and needs the opportunity to return the mouse to the popup before mouseup to avoid dismissing it.

What do folks think?