All native key events come here first: https://searchfox.org/mozilla-central/rev/26790fecfcda622dab234b28859da721b80f3a35/widget/cocoa/TextInputHandler.mm#1883-1892 Therefore, you can check the OS's behavior with `MOZ_LOG=KeyboardHandler:3,sync` (see also [about:logging](https://firefox-source-docs.mozilla.org/networking/http/logging.html#using-about-logging)). (I've not checked recent macOS's behavior, however, we don't have any filter to discard only some `keyup` events.)
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All native key up events come here first: https://searchfox.org/mozilla-central/rev/26790fecfcda622dab234b28859da721b80f3a35/widget/cocoa/TextInputHandler.mm#1883-1892 Therefore, you can check the OS's behavior with `MOZ_LOG=KeyboardHandler:3,sync` (see also [about:logging](https://firefox-source-docs.mozilla.org/networking/http/logging.html#using-about-logging)). (I've not checked recent macOS's behavior, however, we don't have any filter to discard only some `keyup` events.)