Unlike most browser drag & drop operations Calibre adds a 'Boundary Box' to the blue, colour highlight ( a rather pale colour at that ). On releasing the selected book(s), if either the top or bottom line of the rectangular box is missing this will cause one of two things to happen.
With Tags, the drop simply doesn't happen.
With a Hierarchical Tags, the drop will go to the parent Tag instead.
For example;
Code:
Subjects
Archaeology
Astronomy
Astrobiolgy
Astrochemistry
Astrophysics
Aviation
If Astronomy.Astrophysics is the target tag and the rectangle is not fully present then the drop goes to 'Astronomy'.
If 'Astronomy' is itself used as a tag the good luck finding the errors among the other books tagged.
If 'Astronomy' is not used as a tag then the errors can be spotted once you know what happens and corrected.
The rectangle requires great accuracy for correct selection and it therefore slows dropping right down, unlike other systems where positioning anywhere in the highlighted area is OK.
Obviously if the Tag/Subject column is visible the result (or not) can be confirmed but again this slows the process down and that is if you remember to look.
This should be easy, quick and accurate.