Sufficiently recent X servers enable backing store when the Composite extension is enabled, and use Composite to implement WhenMapped support. This causes problems with SDL 1.2 games because they request backing storage whenever the server says it supports it:
According to the code, you're supposed to be able to disable backing store by setting
Option "BackingStore" "off"
in the "Screen" section of xorg.conf. Indeed, xf86SetBackingStore implements this just fine. However, the Composite extension comes along later and overrides the backing store support based on the value of the global 'disableBackingStore' flag. This flag is set in ProcessCommandLine if the -bs flag is passed to the server, but not by xf86SetBackingStore.
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