[GSoC] Support of pluggable theming engines

[Ivan Akulinchev]

>> One thing depends on another. When I worked on my own UxTheme
>> implementation, I found that comctl32 likes to send 0 instead of
>> properly arguments. It angered me.
>
> Do you mean the way comctl32 uses uxtheme API currently is wrong?
> How did you find out it's not using it right way, with proper
> arguments?

I didn't write it uses UxTheme API wrong. Let's say it doesn't use all
features. Thus I can't unfold to the full.

For example, uxtheme provides a great function named DrawThemeText. My
first implementation used Pango because it's the native text engine
for all GTK- applications. Unfortunately I had to refuse this idea
because only a couple of controls used it. Thus the result was very
ugly. But the problem is not limited to the text layout engine. Even
if you want to draw some text yourself, you should probably get the
right color and the right font from GetThemeColor and GetThemeFont
respectively first. At the worst way ask what GetSysColor thinks. But
sometimes comctl32 believes the plain black color should please
everyone. This is crazy, guys!

Back to the words about using the improper arguments. There is a theme
class named LISTBOX. It defines 4 different parts depends on the
scrollbar position. This parts have ids from 1 to 4. If you look at
the file comctl32/theme_listbox.c you see it always sends the
undocumented value 0.

Take a look at the theme class EDIT. One part exists for the
background (EP_BACKGROUND) and 4 parts were generously provided by
Microsoft for the borders around. Let's open the comctl32/theme_edit.c
source file. Firstly it uses only EP_EDITTEXT (probably created for
the text inside). Secondly it doesn't ask UxTheme to draw the
background. Even if you draw the background as a part of EP_EDITTEXT
(as I did), it will be overlapped. This makes problems for such
themes as Ambiance (the default Ubuntu theme).

There is also a cool theme class named LISTVIEW. It provides a lot of
parts for such things as list items (LVP_LISTITEM) which even support
6 states (normal, hot, selected, etc.) Guess how many parts comctl32
uses? Right, only one (the undocumented 0 again). Although a lot of
GTK themes like to use gradients for list items. Even the Windows Aero
theme does it. Therefore I don't believe Windows does it in the same
way (just according to the things I see on the screen).

P.S. I noticed there was a typo in my previous mails. Of course I
wanted to write "flickering" instead of "flipping" saying about the
double buffering advantages. It looks like you understood me right,
just to be sure.