SWI Wimp_PlotIcon and VDU 26

[chris...@bigfoot.com]

Does it strike anyone else as remarkably short-sighted of whoever at
Acorn decided that SWI Wimp_PlotIcon should issue VDU code 26 (reset
text and graphics windows) when called outside the context of a window
redraw loop? Of course, this is undocumented - I had to disassemble
the Wimp in the time-honoured manner to find out. Grr...

I am writing a test harness for some drawing code designed for use in
bona fide Wimp tasks, but the test harness itself merely calls
Wimp_Initialise in order to allow use of Wimp_PlotIcon and
Wimp_TextOp. I have just wasted several hours trying to work out why
icons plotted by Wimp_PlotIcon are not clipped to the graphics
rectangle set up by my drawing code!

I note that the RISCOS Ltd extensions to SWI Wimp_PlotIcon (http://
select.riscos.com/prm/desktop/wimp/wimpploticon.html) do not mention
this pernicious side-effect, although the provision of a flags word
holds out the possibility of allocating a flag bit to mean 'do not
reset text and graphics windows'.

--
Christopher Bazley