How much of the rendering context is clipped on Windows? If it is only a pixel or two, then I strongly suspect this is due to some internal compensations we make in hdi_core to try to keep the widgets as consistent as possible between platforms in both positioning and size.
All our widgets keep an internal state for a "compensation rect" to adjust the (x,y) and/or (width x height); for example, on Mac a ColorWell constructed at (10,20) will physically lie at (10,20) but on Windows it will physically lie at (9,19) for whatever reason. These sorts of inconsistencies vary from widget to widget, so all types have differing compensations from one another.
I believe the compensation rect on Windows for CustomWidgets is either +1 for the y-position or -1 for the height, either of which could explain a 1 or 2 px clipping since you're attempting to perform your own rendering in a frame whose bounds are slightly different in actuality from what you're expecting. On Mac there are no compensations to be made for CustomWidgets (which could additionally explain the lack of clipping on Mac).
All our widgets keep an internal state for a "compensation rect" to adjust the (x,y) and/or (width x height); for example, on Mac a ColorWell constructed at (10,20) will physically lie at (10,20) but on Windows it will physically lie at (9,19) for whatever reason. These sorts of inconsistencies vary from widget to widget, so all types have differing compensations from one another.
I believe the compensation rect on Windows for CustomWidgets is either +1 for the y-position or -1 for the height, either of which could explain a 1 or 2 px clipping since you're attempting to perform your own rendering in a frame whose bounds are slightly different in actuality from what you're expecting. On Mac there are no compensations to be made for CustomWidgets (which could additionally explain the lack of clipping on Mac).