How does a windowing compositor make fancy effects like transparency, window blurring and window animations?
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In the old days, computers had very little memory. So when a window was [on top of another](https://599cd.com/glossary/access/overlappingwindowstabbeddocuments/Overlapping%20Windows.png), the contents of the window underneath were simply lost.
So when you moved the “Items” window away, the “Main Menu” program would get a signal: “Hey, you need to redraw yourself”. And that’d fix the display. You’d usually see the formerly covered chunk of the window start blank then get filled in.
That’s very light on memory, but doesn’t work for fancy window effects, because programs may be slow to respond, which means the effect looks ugly or broken, and the process is generally way too slow to animate anything.
In modern times the system will keep a full image of every window on the screen, so the compositor can apply all sorts of fancy post-processing.
Today this is pretty much universal, the whole idea of a non-compositing desktop is really a relic at this point.
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