Disable, Enable, Unlink and Link Opacity Mask and Opacity Mask Options
(transparency.setOpacityMask) edited the document whenever a target
had an opacity mask, even when every mask already had the flags the
call set. Such a call recorded an undo step, emptied Redo and marked
the document modified.
When every mask a call acts on has those flags already, the call now
returns without editing the document, whether it acts on the
selection, on id or ids, or on the object whose mask is being edited.
The reply is still null, and the journal still records the call. A
call that changes a flag on any of the masks is still one undo step.
One line in the evidence list of docs/target-app-parity.md names the
commands.
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