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Detail:
  On previous Wimps, if you chose to disassociate the Wimp's physical colour
  mapping from the Wimp palette (using SWI Wimp_SetColourMapping), then
  subsequent calls to Wimp_SetPalette would write over the physical colour
  mapping instead of the Wimp palette. *WimpPalette, conversely, had the
  correct behaviour (ie it only ever affected the Wimp palette).
Admin:
  There are actually loads of remaining bugs in the Wimp regarding the
  subtle difference between the physical colour mapping and the palette. Some
  calls try to cause a screen redraw in some circumstances, generally the
  wrong circumstances, and without all the necessary internal Wimp variables
  having been reset (so for example, you might find that window borders end
  up ibeing rendered incorrectly). It probably hasn't all worked properly
  since RISC OS 3.1...

Version 4.53. Tagged as 'Wimp-4_53'