Description
Split from discussion on w3c/webdriver-bidi#446 (c.f. #1790 (comment)).
Not everything in https://w3c.github.io/webdriver/#capabilities has defined matching behaviour under https://w3c.github.io/webdriver/#dfn-matching-capabilities.
If I pass setWindowRect as an alwaysMatch capability, what should behaviour be?
Currently, under "For each name and value corresponding to capability’s own properties:", this falls through out of the "Otherwise" clause because:
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"If name is the name of an additional WebDriver capability" doesn't apply because an additional WebDriver capability is one defined in another spec, and setWindowRect is defined in this spec
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"Otherwise, if name is the key of an extension capability" also obviously doesn't apply
This effectively means that we end up not doing any matching and we also don't fail as a result, and the returned value may well be different to the alwaysMatch value.
From a protocol point-of-view, it is exceptionally surprising that we end up with the returned value being different to the alwaysMatch value.
(Note I'm not super interested in setWindowRect specifically here—and indeed this is arguably a bug—but we do want to allow for other output-only capabilities, and we should define in general what should happen when they are passed in as a capability.)