[Lazarus] csOpaque seems to be broken in 0.9.29

[Mattias Gärtner]

Zitat von Duncan Parsons <D.Parsons at seamsltd.com>:

> Standard way to create 'Transparent' controls is to override the paint
> messages and insert the 'background' you want after inherited and before
> your contained painting. You get the background from requesting a paint
> from the parent of your control.

LM_ERASEBKGND


> Eg in windows you would create an HDC, and send a WM_PAINT to the parent
> control, then based on your own coordinates blit what you want into your
> control. Delphi has some encapsulation around that (as I recall, been a
> few years since I needed to do this).

Sounds complicated.

Opaque painting is in fact an optimized way of clipping and painting  
parents before children.
Remove the clipping and you get transparency.
The problem is that clipping, double buffering and paint order are  
deeply integrated into the widgetsets.


> Now, whether all widgetsets will support this for use in Laz is another
> matter - can we tell a control to paint itself to an arbitrary canvas? I
> have a feeling that OSX might complain, since (from what I understand)
> paint cycles are primarily OS based, and a process draw-request might be
> offensive.. As for GTK.x I couldn't say.

AFAIK carbon/cocoa does not allow that.

The default gtk widget is transparent. But the LCL gtk interface  
cripple this away to be Delphi compatible. One of the gtk3 features  
are several improvements to render widgets to arbitrary contexts.


> But that's essentially the way to do it. You _could_ do it by for going
> the inherited call, and just paint onto whatever canvas you are given -
> but from experience I can say that this can be unreliable: it's worth
> the small overhead of getting the parent background.


Mattias