There may be several ways to make the Markup designer ready.
(Note: publishing required HL properties, is a different issue/branch again)
1) just make them components.
This will also make the MarkupMgr a component, but that one must not be registered.
Also Markups then need a publisher read/write SynEdit property, but the MarkupMgr MUST NOT have that.
A Markup then is kind of owned by the SynEdit and/or the form.
Owner = form, but that means form is responsible for destroying, yet old code assumes SynEdit is, and that also needs to be kept, for compatibility. (SynEdit needs to check, if owner = form)
2) Make SynEdit the owner even in design. (like frames).
SynEdit afaik already has the attributes. It just needs to accept the markup and drop (and forms must reject it).
Investigate gutter parts. they are owned by the SynEdit. (look at an lfm)
3) Similar to 2, but do not add Markup to the component palette.
Instead have a special property editor. (like gutterparts.)
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I prefer 2 and 3 over 1.