When I open the iCloud Photos folder with Directory Opus I'm not able to see thumbnails and I can't open files that haven't been downloaded to the disk.
The thumbnails aren't displayed even if Explorer already created them.
When I open a file that has not been downloaded I see the download notification but it stops at 100% and never opens the image. Only when I click "Cancel Download" at 100% Opus opens the file.
Ok, so after playing around with this for a while it seems more of an issue with iCloud than with Opus. Other programs are affected as well.
As a workaround it is possible to force iCloud to download all images by right-clicking on the iCloud Photos folder and select "Always keep on this device" (only visible in Explorer, Opus does not show this option). Unfortunately, this affects only existing files so any new pictures will again be virtual only.
It seems this option is set as a non-standard file attribute 0x80000. After this flag is set iCloud automatically downloads the file. I was able to set this with PowerShell, but I guess it would be possible to write a button for Opus to set it for individual files or iterate over all files in the folder.
(Get-ChildItem <FileName>).Attributes = (Get-ChildItem <FileName>).Attributes -bor 0x80000