[Mono-winforms-list] Scrolling performance isues

[Carlos Alberto Cortez]

Hey,

The new code that detects not visibles areas of the window to scroll
(obscured by other mwf windows or x11 top level ones), although working
fine, is somewhat slow, as some people have noticed.

After some research, I found that getting all the child windows for the root
window (using XQueryTree on RootWindow, this is, top level windows) is the
hot spot, and was causing the slow down.

And after some more research in other graphics tool kits, found that:

* Most of them make use of GraphicsExpose event, handling it by directly
generating expose events or invalidating the area pointed by GraphicsExpose
(Gtk+ as far as I know, and also Ivan told me other frameworks do that).

And also, regarding our code:

* When using composite (Xgl or similar) it's not necessary to do this
detection, since the window manager (*it seems*) is doing something that
somehow already knows which areas need to get an expose event (in other
words: only mwf overlapping detection is needed, not for x top level
windows).
* The new code, at least in my laptop, without using composite, seems to
work fine (no performance lost), and don't know if it's because XQueryTree
on RootTree returns a minor number of windows (61 with no composite, instead
of 89 with composite, using 5 terminals and a gtk+ application, for
example).

So, it seems that we should actually handle GraphicsExpose (which all it
involves) *or maybe* try to detect if we are using composite - if we are,
simply don't use this new code, but just do simple calculations, and if we
are not using it, do the detection, since it won't harm the performance.

But I woould like people to tell me how this code (2.0 branch or trunk)
behaves for them, in both cases (since I remember that Ivan was having
performance problems with the new code even without using composite).

Hope it's clear ;-)
Carlos.
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