directWrite 1px offset between redraws

Using the equal AttributedString to draw a string with justification verticallyCentred, the text jumps sometimes up and down in the range of 1px, even if its coordinates doesn’t change.

Also between redraws, so if i only redraw the right half of the text, the other part has sometimes a y-offset of 1px.

After first tests, it seems changing the IsPixelSnappingDisabled-callback to true removes the symptom.
But i guess somewhere a hidden-parameters (uninitialised variable?) is the real culprit.

The issue reminds me on the long-running bug, which has caused wrong text-layouts on windows, maybe something is still missing…

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