winex11: Fix alpha blending in X11DRV_UpdateLayeredWindow.

[Sebastian Lackner]

On 07.02.2017 17:08, Alexandre Julliard wrote:
> Sebastian Lackner <sebastian at fds-team.de> writes:
> 
>> @@ -2627,14 +2627,12 @@ BOOL CDECL X11DRV_UpdateLayeredWindow( HWND hwnd, const UPDATELAYEREDWINDOWINFO
>>      {
>>          IntersectRect( &rect, &rect, info->prcDirty );
>>          memcpy( src_bits, dst_bits, bmi->bmiHeader.biSizeImage );
>> -        PatBlt( hdc, rect.left, rect.top, rect.right - rect.left, rect.bottom - rect.top, BLACKNESS );
>> -    }
>> -    ret = GdiAlphaBlend( hdc, rect.left, rect.top, rect.right - rect.left, rect.bottom - rect.top,
>> -                         info->hdcSrc,
>> -                         rect.left + (info->pptSrc ? info->pptSrc->x : 0),
>> -                         rect.top + (info->pptSrc ? info->pptSrc->y : 0),
>> -                         rect.right - rect.left, rect.bottom - rect.top,
>> -                         (info->dwFlags & ULW_ALPHA) ? *info->pblend : blend );
>> +    }
>> +    ret = BitBlt( hdc, rect.left, rect.top, rect.right - rect.left, rect.bottom - rect.top,
>> +                  info->hdcSrc,
>> +                  rect.left + (info->pptSrc ? info->pptSrc->x : 0),
>> +                  rect.top + (info->pptSrc ? info->pptSrc->y : 0),
>> +                  SRCCOPY );
> 
> BitBlt is not guaranteed to set a valid alpha channel.
> 

Could you please clarify in which situation this code will not work as expected?
I have checked the relevant code, but so far I have not found any reason why it wouldn't work here.
Also, it works perfectly fine with the test app written to demonstrate this bug
(which also uses alpha channels). Am I missing something?

Regards,
Sebastian