If image is a transparent one and any foreground color in that image matches "settings->viewing->main window color" then Irfanview automatically changes background color for displaying that image modified like 50% of what it is set (if necessary automatically reload image to reflect background color change).

For example;
- Settings have main window color set to black.
- Picture displayed has transparency.
- Picture displayed has black in foreground color (displayed color).
For such a picture;
- display background color will be switched to 50% less black (aka gray).
- Settings is going to have main window color not changed (it will still be black).
If next picture is transparent one and any of foreground color(s) does not have black then it will still be displayed in black main window color as in settings.

Probably such a feature can be included in options to allow people turn on/off upon any need.

Reasons for request:
- That will help people mostly dealing with transparent images not to play with background color each time there is non-visible or partially visible images.
- Sometimes you cannot understand that part of the picture is not visible because that small part has same color as main window color in settings.

I believe, Pictures application in Windows 10 does such a thing already. Cannot be sure though.

Thanks & regards,