Please don't require horizontal elements to total 100% of screen width
It drives me up the nearest wall when I change the size of one of several elements in a horizontal row and all the others change size to total 100% of screen width. This adds a lot of extra time to screen design work.
Simple example I just ran into: I placed a slideshow onto its own row and set its width to 15% of screen size. It was on the left edge of my screen. So, I put a spacer to the left of it. It was automatically set to 50% of screen width, and the slideshow jumped to 50% of screen width! I set the spacer to 10%, which is what I wanted. The slideshow jumped to 90%! So I changed the slideshow back to 15% and the spacer jumped to 85%! So, what I need to do in general is to plan my horizontal row first, placing all of the elements I am going to need without regard to width, and knowing that I am going to need elements that total 100% of screen width. Then, I can work from left to right, setting the widths to their desired settings. That is just plain ridiculous!