Guys, have you noticed that sket used Part Design Pads to create the surfaces? sket must use 0.13.1828. Just try to edit one of the pads, it will throw an error.
sket: the purpose of the Part Design workbench is to create solids. Making surfaces is not allowed anymore in the current v0.14 development version which is almost ready to be released as the new stable. To extrude surfaces you should use Part Extrude
A surface trim tool sure would be nice.
An alternative way at the moment would be to use the Part Section
tool to create an intersection edge on the upright surface, then use Shape Builder to create vertical edges then make a face of this.
looo, your solution is good as well and less of a hassle. Though it's a pity that all these alternatives don't create a parametric surface.