On mobile, if I long press the reactions button, the reaction selection pop-up appears for a second and then disappears, which does not give enough time to select a reaction. I long tap again, but this time the pop-up does not appear at all.
I end up reacting with 
for every post when I am on mobile, which is not an appropriate reaction in some cases.
For me on iOS mobile you long press and then move your finger away and the reaction selector stays open. It may have to do with your mobile OS version, sorry.
Yeah, possible. I am on Android and Brave browser.
FYI, a year ago I did a load of tests on “responsiveness” for that kind of thing on several browsers on Win 10, Android phone 8, Chromebook (using Android apps).
I abandoned those tests.
Why? Because I could not get my head around the multiple factors that might be in play.
I could not work out what was my f*k up and what was a systemic issue.
Not dissing your point/issue at all!
I just think there is less reliability/consistency than is advertised, perhaps?
Dunno. How would one really sort this (your kinda OP) out, definitively?
Just a comment
TT
I also have experienced the same issue in my android phone
I haven’t used Brave, but it seems to work fine for me on Firefox and Chrome (Android 13).