This is somewhat related to https://bugs.kde.org/show_bug.cgi?id=426704.
STEPS TO REPRODUCE
1. Use Plasma with default settings
2. Fill the disk until less space is available than the trash size
When on the trash page, dolphin shows all trash size as available space in trash.
It should consider the already occupied space on the drive that will not be available for the trash.
This behavior might cause unexpected effects when a non default option for treating trash files when the trash is full is selected (like deleting them), although it might be impossible to get a full trash in this state.
Created attachment 133335 [details]
Free space on root partition
Created attachment 133336 [details]
Free space displayed for the trash
Wait, does moving a file to trash actually copy the file? That itself would be a bug.
Did you comment on the right bug? If yes, I don't understand what you mean with copy and how that is related to this report.
Moving a file doesn't cause new block allocations, so the available space doesn't change. Copying does cause block allocations, and the size available for the trash would indeed need to take into account the available space for these.
While writing my answer I finally got what you meant. I had the misconception that trashing files would occupy new space which is not true (and also solved for external drives recently). So the space is indeed displayed correctly.
If that is the case then https://bugs.kde.org/show_bug.cgi?id=426704 (where I got the inspiration from for this bug) has been wrongly addressed, correct?
I am too tired to investigate/read properly, but if I understand this report correctly, it is NOTABUG.
The Trash size is a limit to avoid that all space is occupied by trashed files.
Imagine you have configured your Trash size to 100GB, with 12GB already used there. Disk has only 44GB space left. Now you are able to move an additional 88GB of files to the Trash, without changing the 44GB free space figure.
Regarding bug 426704, you are probably correct that the fix is wrong.
Agreed, so closing this bug.