Fix ComboButton click on webkit mobile (iOS and android).
Set dijitOffScreen class on <input> so dojo/touch doesn't squelch the
click event generated from _ButtonMixin's call to this.valueNode.click().

Fixes #17693.

(cherry picked from commit d3739cfb8859b990c65777250e4507c67018233f)
fix over-eager setting of aria-invalid="true", fixes #17928

(cherry picked from commit 467a1e21899878f3fb9ec29ac1feb5df343ce684)