A few errors (corrections sent) but worth a read. It was the first computer shipped containing an ARM, the ARM 2. Ignore the crummy looking screenshots of Arthur; the WIMP was usable in Arthur for single-tasked applications but a very-nice co-operative multi-tasking desktop came with RISC OS 2.0.
It had features I still miss today, e.g. drag a scrollbar thumb with Adjust (X's Button3) instead of Select (B1) and you were panning that scrollbar and its partner in the other axis. Adjust often did similar to Select but with a tweak, e.g. the pop-up context menu with Menu (B2) could have an item selected with Select and would disappear but use Adjust and the menu, possibly several deep, remained opened. How often today do you have to re-navigate to a nearby place to select a second item? Select on a scrollbar's up arrow scrolls up, Adjust down; no need to move to the other end of the bar. Same goes with paging by clicking to the side of the thumb.