Working from the OSLib API definitions:

RISC OS 4.20 introduced:

* Message_ToggleBackdrop           &400d1
* Message_ScreenEdgeNotification   &400d2

RISC OS 4.32 introduced:

* Window flags 2 bit 1: UseExtendedScrollRequest
* Scroll_Request event directions: +/- 4: SingleExtended
                                   +/- 8: DoubleExtended
* Scroll_Request event block: word at offset +40 is an icon handle
* Wimp_TextOp 1 flags bit 31: SizeCovered
