Panel: when floating, de-float when window touches it
A floating Panel now de-floats and becomes opaque whenever a window touches it. This also allows for a thinner Panel when a window is maximized (a normal Panel is 44 px; before this patch a de-floated one was 60, now it's 52px) and that should allay complaints about excessive margins). BUG:455538 BUG:455483 BUG:455323 FIXED-IN: 5.27