What I was trying to do is explain the rationale behind what you see in the Dialog.
The general case for such a Terrace Door is that the Window has Sill and the Door does not, therefore the vertical lengths of the two sides are not equal. This is why setting the Reveal is disabled by default, because the Reveal would not be correctly displayed (save the special case of the two sides being of equal height).
What you are trying to do is a specialized case for which this object was not meant.
But I also tell you a secret now that I dug into this thing:
I checked into the object and the last parameter it has on its parameter list has the name "Reveal Tab page Control". It actually lets you enable/disable the Reveal Tab page. Just by experiment, if you set a value of 0 or 1 then the Reveal Tab page will be enabled. To do this you will have to either extract the LCF container so you can open the library part for editing or you have to save it under a new name and edit that.
When you open it for editing, uncheck the Hide button before its name and save it, so it will appear in the Parameter list in the Window Settings Dialog.
Then set the value to 0 or 1, click OK to close the Dialog and then Open the Dialog again. It seems this close and open is needed for any change in the value of this parameter to take effect.
But keep in mind that the Reveal will be fine only if the vertical height of the Window and the Door are the same.
If they are not there will be a reveal towards the Door on the side of parapet below the Window.
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