How can I create a trapezoid that can be resized horitonally and vertically, but maintain set angles?

hi!

that’s how i’d do it within a shape layer. please look at attached screenshot.

notes:

a favorite expression lets me define and scale a rectangle from one of its corners:

s = thisProperty.propertyGroup(1).size;   [s[0]/2, s[1]/-2]

here i divide the X value by 2 and the Y value by -2, so that the rectangle grows to the right and upwards, sticking always to its bottom left corner.
(this is applied to the “Position” property inside a “Rectangle Path” object, and refers directly to the adjacent “size" property)

now duplicate the rectangle and group the second copy into a shape group, allowing you to skew it independently under the group transform attributes.

i added “Merge Paths” in “Add” mode, to create one unified shape out of the two rectangles. note that this “Merge” needs to be placed under the two other elements, and make sure it is not inside the secondary group created for skewing, but outside, in the main group (here automatically named “Rectangle 1”.

with the orange arrow i marked the Y value of the second rectangle, which you can now scrub and change, and assign any value to - maintaining all the angles.

another useful thing - i use “UU” as a keyboard shortcut, which reveals all attributes of a layer, which have been altered by me from their default values, which is what you see in this screenshot.

hope this helps!