Alerts and Message are shown on the screen by ScummVM using GUI dialog
boxes. They may have clickable buttons on them.
In Director, it may happen that the event of clicking or pressing the
button on the GUI dialog box (mouseUp, keyUp) get recorded as a movie
event, which may cause unpredictable change in the lingo script.
Prevent this by disabling all event processing in the Director movie
using a flag _inGuiMessageBox, which is set by the function that calls
the alert or dialog box and cleared once the dialog box dissappears
In this case that is the `b_alert()` function
Malhar
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