On Nightly 56.0a1 (2017-06-22). Steps to reproduce: 1. Click the "Downloads" button in the toolbar. 2. Click the same button again to close the dropdown. Expected results: On both actions, there is an instant visual feedback on the button itself. Actual results: 1. On click-to-open, the button gets a darker background instantly. 2. But on click-to-close, the button returns to the lighter background after a visible delay. I haven’t timed it, but it seems the "open" style on the button is removed only once the dropdown’s fade-out animation has completed. Or maybe it’s hardcoded to a 300ms delay or something, because it feels that the button’s style delay is longer than the dropdown’s fade-out animation. (This is true with every navigation toolbar button which has a dropdown: downloads, bookmarks, overflow menu, application menu, and add-on buttons.) To me, this makes the button feel like it’s not very reactive, and makes you doubt that the click was registered and you wonder if you have to click again or click more forcefully. :P

So the expected result is remove the "open" style of button immediately when close animation begin?

Yes. Or maybe something else, but the idea is to have immediate visual feedback, like the previous theme(s) did. For instance with the default mac theme and for the main menu icon, when performing a "closing" click: 1. on mousedown: the icon gets darker. 2. on mouseup: the button background switches from the "open" state to the "hover" state. Since perceived UI responsiveness is a major goal of Photon, I think it’s better to not lose this kind of instant feedback.

(In reply to Florens Verschelde from comment #2) > Yes. Or maybe something else, but the idea is to have immediate visual > feedback, like the previous theme(s) did. Is this really a regression? That seems surprising. Can you use mozregression to work out when this broke? Because I doubt anything changed here, besides the actual :active/[open] styling of the button.

Hmm I think you’re right, I tested again on Firefox 54 and the navbar buttons do have the delay. Not sure what confused me in my tests yesterday. So we’re back to my original report: IMO the delayed feedback feels sluggish, and it might work well with the Photon design goals to make it instant instead. Of course that’s just me, and maybe no one else will get a feeling of unresponsiveness because of that; but I do think it might have an impact.

Given earlier testing: probably not a regression, but can still be a minor "perceived performance" issue worth addressing at some point.