YAMAMOTO Mitsuharu wrote:

> It seems to be necessary to look into the tRNS chunk data for paletted
> images.  Again, W32 would need some more work.

Thanks, I confirmed that this fixes the misdisplay for me as well. I attempted 
to port it to w32 (as well as to unusual libpng builds that do not define 
PNG_tRNS_SUPPORTED) and installed the attached into master.

With this patch on Ubuntu 18.04.3 I always see the first image shown in your 
tRNS.pg screenshot <https://bugs.gnu.org/37153#56>. So it may be that more work 
needs to be done to support other faces. Still, this patch is an improvement so 
it's worth going in.