100% vertical and horizontal panels overlap
Submitted by Andrzej
Two panels, one vertical and one horizontal, attached to the (e.g. left and top) edges of the screen, compete (overlap) for the corner of the screen.
Making one of them shorter is difficult because the length property is scaled in percents rather than in pixels. It almost always leads to a gap between panels.
Ideally, this should not be necessary, 100% could for example denote an available space, rather than the screen size. The panel with a lower number could be allowed to take the whole edge of the screen and panels added later would have to respect struts set by earlier panels.