I never saw that behavior (expending folders when hover with a file to move) and that lies beyond my interests. Ironically, that behavior was harmful for me when I used Mac OS. There were several times that I released the left button 'simultaneously' with the Finder opened the destination folder, so that my file got moved to some location inside the destination folder, which wasn't my intention. I usually have two Dolphin's windows opened with the detail view mode in each workspace.
The things I like in Dolphin are of another sort. This is designed closer to the well-established and reliable logic of working in a command line of console windows. In short, I feel some harmony in this as in good music. There are a lot of keyboard shortcuts like F3 to split or F2 to rename, which accelerate work. I also like the pop-out window suggesting a choice either to move or to copy a file, and I often use the build-in command line (F4). I'm using KDE in my everyday work though I'm not a coder.
Anyway, let me thank you for your curiosity -- at times it leads to progress.