A GUI::Widget can now set an optional content margin (4x0 by default.)
Pixels in the content margin will be ignored for hit testing purposes.
Use this to allow frame-like widgets (like GUI::Frame!) to ignore any
mouse events in the frame area, and instead let those go to parent.
This allows GUI::Splitter to react "sooner" to mouse events that were
previously swallowed by the child widgets instead of ending up in the
splitter. The net effect is that 2 more pixels on each side of a
splitter handle are now interactive and usable for splitting! :^)
We were previously cheating by setting the entire splitter's background
color to the hover highlight color. This looked goofy whenever there
were transparent widgets inside a splitter, since the highlighted color
would shine through when hovering.
This was especially noticeable in SystemMonitor, which now looks much
better. :^)
