Problems with expose_event->region patch

[Alexander Larsson <alla lysator liu se>]

I have an almost working patch for getting the full region in expose
events. Unfortunately the small problems left may be showstoppers...

The core problem is that mixing event->area and event->region in expose
handlers just doesn't work when handling NO_WINDOW children.

Here is an example that shows the problem.

Consider a window with a vbox, two frames, and labels in the frames:

 +-frame1---+
 | Label1   |
 +----------+

 +-frame2---+
 | Label2   |
 +----------+

Note that all widgets (exept the GtkWindow) are NO_WINDOW, so there is
only one X window.

Now i change the widgets:
  gtk_widget_queue_draw (label1);
  gtk_widget_queue_draw (label2);

This invalidates the area of the labels, and this invalidation will be
compressed into one expose (event->region being two rectangles). The
expose event will be sent to the GtkWindow.

GtkWindow starts by clearing the exposed *area*, then propagating the
expose to the frames. This means that label1, label2, and the rectangle
between them, containing part of the frames are cleared.

But when propagating the expose to frame1 the intersection of the
event *region* and the frame1 rect is just the label1 rect, so frame1
doesn't repaint the lower part of the frame which was previously cleared.
Even though both GtkWindow and GtkFrame just uses event->area.

The problem here is that the exposed area changes with event->region
support, since:
intersect (rect(expose_region), window_rect) != rect (intersect (expose_region, window_rect))

So, "clear area, propagate to NO_WIN child, render area" doesn't work.

"clear region, propagate to NO_WIN child, render area" doesn't work
either, since the NO_WIN child depends on having the area it paints being
cleared.

So, for this to work we need to do "clear region, propagate to NO_WIN
child, render region". This is probably not an optimization though, since
we will need to use a lot of complex clip regions all the time (not to
mention all the gtk_paint_xxx need to take regions instead of rects, and
breaking all expose handlers in existance).

In practice we may be able to get most of the benefits of expose regions
by doing "clear area, propagate to NO_WINDOW childer the old way, render
area or region (they are equal now)". Then non-container !NO_WINDOW
widgets can use use expose region to optimize their redraw.

The case of leaf !NO_WINDOW widgets is problably the case where using
regions to repaint is most useful anyway. This could be drawing areas used
as canvases and whatnot.

Comments?

/ Alex