Window activation events on Windows-10 don't seem to be reliable as it's possible for Alt-Tab to trigger WM_ACTIVATE on a window when switching away from it. As detecting the keys which are held relies on a valid active state - this meant Alt could become stuck when using Alt-Tab to switch between windows. Disable reading modifiers on activation for WIN32, activating the window now clears modifiers on WIN32. This isn't ideal as held modifiers wont be detected, re-introducing the error reported in #40059.