Under Linux, Firefox, Chromium and GNOME Web (which is WebKit-based; a.k.a. Epiphany) all have Tab cancel but not Esc.

The GTK 4 behaviour feels very much like a bug. I can reproduce it in gtk4-demo and in gtk3-demo. It leads to obviously-nonsensical behaviour, including messed up window focus. I’m tempted to file a bug report, but haven’t had such productive experiences with filing things on GNOME historically as with Firefox or even Chromium.

Why should Enter being a different key be a different discussion? They’re both used to activate buttons, and one is behaving like every single other keyboard interaction, while the other isn’t. (Related: if a link is focused, Enter activates it, Space doesn’t, just doing the normal scroll-down-one-page.)