macOS: Should never encounter KeyData when transitMode is rawKeyData. #87339
Description
Happens after the keyboard event refactor landed.
[ERROR:flutter/lib/ui/ui_dart_state.cc(209)] Unhandled Exception: 'package:flutter/src/services/hardware_keyboard.dart':
Failed assertion: line 787 pos 16: 'false': Should never encounter KeyData when transitMode is rawKeyData.
It happens in the following scenario: Register macOS global hot key, on hot key create new flutter window/engine and show it. However I assume this would also happen in different multi window scenarios (and is not necessarily tied to a hot key).
What happens is that cocoa immediately sends NSEventTypeFlagsChanged to the new window with event.modifierFlags == 256, which basically means no modifiers.
However the code in FlutterChannelKeyResponder does this:
case NSEventTypeFlagsChanged:
if (event.modifierFlags < _previouslyPressedFlags) {
type = @"keyup";
} else if (event.modifierFlags > _previouslyPressedFlags) {
type = @"keydown";
} else {
// ignore duplicate modifiers; This can happen in situations like switching
// between application windows when MacOS only sends the up event to new window.
return;
}_previouslyPressedFlags is 0, so this always generates key down event even though there aren't any modifiers present. Which in turns sets _transitMode in HardwareKeyboard to KeyDataTransitMode.rawKeyData, because this is sent before any actual ui keyboard event. After this the keyboard no longer works in window resulting in message above.
Potential solution would be either initializing _previouslyPressedFlags to 256, or check if event.modifierFlags > 256 before generating key down event.