Long (500ms) Pulse Extender
As much as it pains me to say this, a CMOS 555 will do exactly what you want.  The standard circuit will give a constant output pulse width for any input pulse shorter than the timeout period.  If the input still is low, the output will remain high until the input goes high; then the output will go low immediately.  Retriggers caused by switch bounce during the timing period are ignored.  The Micrel MIC1555 is in a 5-pin SOT-23 package.  1 chip, 1 R, 1 C (plus decoupling).

https://www.microchip.com/wwwproducts/en/MIC1555