This issue does not describe a bug of Flutter. This describes a bug of Win32.

When the user releases the right shift key, the Win32 embedding does not receive any key up events. It's not about the fault of the embedding; instead, WndProc, which directly connects to the system, does not receive any messages at all.

As a consequence, the right shift key is stuck at a pressed state (see #75675).

The hardward keyboard API solve this problem by checking key states at every key event, meaning that a synthesized key up event of the right shift key will be dispatched at the next event. The raw key event API, however, does not have such checks for now.