Eyes Above The Waves

Platforms like X and Win32 support trees of native windows. For example,
an application might have a "top-level" window containing child windows
representing controls such as buttons. In browsers, child windows are required
for many kinds of plugins (e.g., most uses of Flash). Managing these child
windows is quite tricky; they add performance overhead and the native
platform's support for event handling, z-ordering, clipping, graphical effects,
etc, often is not a good match for the behaviours required for Web content,
especially as the Web platform evolves. So I'm working hard to get rid of
the use of child windows and I've made a lot of progress --- more about that
later. However, we're stuck with using child windows for plugins, and recently
I've been grappling with one of the hardest problems involving child windows:
scrolling.

It's very hard to make scrolling smooth in the presence of child windows
for plugins, when all other child windows have been eliminated. In general
you want to scroll (using accelerated blitting) some sub-rectangle of the
document's window (e.g., an overflow:auto element), and some of the plugin
windows will move while others won't. You may want to change the clip region
of some of the plugins to reflect the fact that they're being clipped to the
bounds of the overflow:auto element. The big problem is that to avoid flicker
you want to move the widgets, change their clip regions, and blit the contents
of the scrolled area in one atomic operation. If these things happen
separately the window is likely to display artifacts as it passes through
undesired intermediate states. For example, if you blit the scrolled area
first, then move the plugins, the plugin will appear to slightly lag
behind as you scroll the window.

On Windows, href="http://msdn.microsoft.com/en-us/library/bb787593%28VS.85%29.aspx">
ScrollWindowEx with the SW_SCROLLCHILDREN flag gets you a long way. But
on X, the situation is dire. X simply has no API to scroll the contents of a
window including child windows! Toolkits like GTK resort to
heroic efforts such as

guffaw scrolling to achieve the effect, but those techniques don't let
you scroll a sub-rectangle of a window, only the entire window. So for Gecko
I have had to come up with something new.

Basically I want to use
XCopyArea and have it copy the contents of the window *and* the contents
of some of the child windows in the scrolled area, and then move the child
windows into their new locations and change their clip regions (I'm using
XShape for this) without doing any extra repainting. I tried a few things that
didn't work, before I hit a solution:

1. Hide (unmap) all child windows that are going to be moved during the scroll
   operation
   
2. Do the XCopyArea
   
3. Set the clip region and position of all relevant child windows
   
4. Show (map) all child windows that we hid in step 1
   

By hiding the child windows during the XCopyArea, we trick the X server into
treating the pixels currently on-screen for them as part of the parent window,
so they are copied. By moving and setting the clip region while each child
window is hidden, we also avoid artifacts due to a child window briefly
appearing outside the area it's supposed to be clipped to. It *does* mean that when we scroll a window containing plugins, expose events are generated to repaint the contents of the plugins' child windows, so scrolling is slower than it could be. We might be able to avoid that by enabling backing store for the plugin windows.

It's somewhat tricky to implement this. We have to compute the desired
child window locations and clip regions, store the results in a collection,
and pass that collection into our platform window abstraction's Scroll() API
so that the scrolling and widget configuration can happen as a unified
operation. But I've done it, and it works great on my Ubuntu VM. I'm not
100% sure that it will work well on all X servers and configurations; that's
something we'll need to get good testing of.

I do wonder why X has never felt the need to grow a genuinely useful
scrolling API. Oh well.