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Detail:
  UpCall 6 is documented as accepting a pollword which can be scanned for
being non zero to return control to the process requesting to sleep.  This
was not being used within the taskwindow module, there being a comment where
it would have been used suggesting that this would give the taskwindow too
high a priority.  What this means is that if given a pollword that is already
nonzero, the task would receive so many polls as to really make the desktop
juddery.  The solution is that if the pollword is non-zero on entry,
Wimp_Null is unmasked rather than passing the pollword.

  This means that UpCall 6 can be used either as a yield, to say 'let other
tasks have some CPU now' (by passing a pollword known to contain nonzero) or
as sleep to say 'wake me up when the pollword becomes non zero, and don't
consume any CPU time until then' (by passing a pollword that will only become
nonzero upon the occurance of a specific event).

Admin:
  Tested on RPC.

Version 0.65. Tagged as 'TaskWindow-0_65'