You probably want to use a drop zone in this case. I'll get to it… last or thereabouts.
How to build a window frame:
Draw a Rectangle/Square. Call it ContentArea. Fill it with a color and turn off the Outline.
Duplicate the shape. Call the duplicate ContentBorder. Turn off the Fill and set the Outline Color (I'll leave the choice of width up to you.)
To ContentBorder > Shape > Size add a Link behavior and apply ContentArea to it.
To the Position parameter add a Link and apply Content Area to it.
Create a new Rectangle (call it WindowFrame and link the Size > Width to ContentBorder > Width and the Size > Height to ContentBorder > Height. Set the Height Offset to 50.
Set the Fill to your Titlebar main color and the outline to Match Content Border (you might just want to link the two.)
Add a Behaviors > Basic Motion > Align To behavior and apply ContentBorder to it. Set the Align and To parameters to Bottom.
Move WindowFrame to the bottom of the Layer Order.
Duplicate ContentBorder and to the copy, change the Outline Color to Black, the Brush Opacity to about 45% and the width to 1. Open the Link behavior for the Size and at the bottom, set Width Offset and Height Offset to -3 (the actual value will depend on how wide you make the window border - so about half that.)
Select this duplicate ContentBorder copy and type Command-Shift-N to create a New empty group.
Window Control buttons:
Select the New Group layer and —
Draw a small square. Size it to fit in the Titlebar area vertically as you want it.
Duplicate the square twice. Arrange the 3 rectangles in a horizontal row as you want it.
You can use these 3 unicode characters to overlay the squares: ✕ ▢ ⎽ (the square will need a different Font Sizes to look visually the same size as the other two -- I used 17 for the ✕ and ⎽ and 14.8 for the box ▢). Line these three characters over the window control boxes.
To the Group (layer), add an Align To behavior. Apply WindowFrame (rect) to the Object with Upper Left for both Align and To. Then Offset X and Y to line up the shapes in their place.
Select the main Group and add a Text Object ("window title"); Alignment = Center . Apply Align To _ Object Window Frame _ Top Top. Adjust Offset Y to bring it down into the middle of the Titlebar region.
For the "Next" area - do basically the same as the window controls; Align To Upper Right/Upper Right; adjust offsets.
The Drop Zone:
Select the ContentArea Rectangle. Go to Add Object > Drop Zone. Right click on the Drop Zone and Add Image Mask. Apply ContentArea. Go to Properties > Transform > Position and add a Link behavior and apply ContentArea to the Source Object.
Resizing, Position, etc.
Everything should be under control of ContentArea - the Size Parameters, the Position of everything. This is the object properties you need to publish to control where onscreen and size, etc.
You can darken up the window control square character by simply duplicating it in place. I used this unicode character for the hamburger menu: ≣
How you color you window will be up to you, just keep in mind that the ContentArea and WindowFrame outline colors should match.
Let me know if you run into trouble...
PS - You can publish the Pan and Scale parameters of the Drop Zone, but first you'll have to add an actual image to the drop zone for those to **appear** (to have access to). Afterwards, just reset the Drop Zone to "Drop Zone" and delete the image you used.
