It happens that Chris Hogg formulated :
On rare occasions I have left our (upright) freezer door not fully
closed. I doubt that it got significantly less cold inside, and the
compressor probably worked hard, but when I came to open it the
following day, it was a mass of icicles and frozen condensate,
requiring a total defrost to clear it.

As was mine, which is why I decided a solution was called for..