Strange problem with "while loop"

1

In a game, I want to show hints appear on the screen for 100 miliseconds.
What is wrong with this function? “txt._visible=true” line never executes.

txt._visible = false;
btn.onPress = showHint;
function showHint() {
t = getTimer();
while ((getTimer()-t)<100) {
txt._visible = true;
}
txt._visible = false;
}