If I open the images from several sub folders when there are a lot of files pressing enter will not go to fullscreen mode until all the thumbnails have loaded. Spacebar will also not advance to the next image until all the thumbnails have loaded. In older versions of IrfanView this did not happen. If you go to the menu View > Fullscreen mode it will open fullscreen mode and spacebar will advance the image.
Using version 4.60 64-bit
Using version 4.60 64-bit
