Do not create image masks in ImageList when using 32-bit color depth (or provide a way to disable it) #14171
Description
.NET version
10.0
Did it work in .NET Framework?
Not tested/verified
Did it work in any of the earlier releases of .NET Core or .NET 5+?
No
Issue description
See this article from 2004 with a fix: https://www.codeproject.com/articles/Adding-and-using-32-bit-alphablended-images-and-ic
Specifically, when using ImageList = new ImageList { ColorDepth = ColorDepth.Depth32Bit }; the image list still creates a mask which causes poor anti aliasing on transparent pixels in the bitmap.
The fix in that article is to manually call windows ImageList_Add without the mask. I've tested this in .NET 10 and the fix works and fixes icons having bad transparency in trees/lists/tabs.
before:
after:
look closely, you can see gray pixels when not using the fix
Steps to reproduce
Create a imagelist, add it to a treeview, and add a bitmap image that has transparent pixels (not fully transparent)