On Android pre-R with a fixed nav bar stacked below the IME (e.g.
Huawei EMUI 12, 3-button nav), the prior formula
`systemWindowInsetBottom - stableInsetBottom` reported only the IME
proper and dropped the nav-bar strip. Consumers max-ing the value
against viewPadding.bottom under-shifted by exactly the nav-bar
height, so the composer's bottom edge ended up behind the IME's top
edge by ~stableInsetBottom.
iOS reports keyboard frames spanning all the way to the bottom of
the screen (covering the home-indicator area), so the cross-platform
consumer pattern `max(keyboard.height, viewPadding.bottom)` already
assumes that semantic. Pre-R now matches: when the IME is open,
publish the full `systemWindowInsetBottom` (nav bar + IME);
otherwise 0.
R+ branch untouched — `Type.ime().bottom` typically already includes
the nav-bar area on devices where the IME extends to the screen
edge. Left a TODO on the R+ path for HarmonyOS R+ with a fixed
nav bar; not currently testable.
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