Regarding creeping/dangling thumb:
Generally speaking, maybe ~> 90% of the time one could be playing passages or phrases whereby the fretting fingers are curled, the thumb is near the middle of the guitar neck on the back-side, and the thumb and middle finger form a C-shape sandwich with the guitar fretboard and neck in-between.
The fretting wrist and forearm form a straight line for maximum comfort, the fretting wrist is not bent which can cause strain when its bent, you want to avoid that.
Now, try and fret a D major chord in first position and attempt to maintain all of the typical posture notes for the fretting hand while keeping the thumb in the middle of the neck.
My guess is that you will have to bend your wrist, feel discomfort, extra strain etc if you want to try and keep the thumb in the middle of the neck.
In order to fret the D major chord and keep the forearm and wrist straight for maximum comfort you will have to relax and allow the thumb to move away from the middle area of the neck and probably somewhere between middle of the neck and the edge.
When the thumb is in this location, sometimes at an angle the upper edge of the thumb may be seen by an observer from some angles, but the entire first joint of the thumb is not coming over the edge poking out like a finger puppet
In this kind of scenario, there's nothing to worry about, its a natural adjustment.
However, if the thumb is not anchored somewhere on the back of the neck and is entirely exposed/free, it could be an indication that the rest of the hand is not positioned and holding the neck correctly, if one is trying to play a barre chord, there will likely be lots of buzz, incorrect application of pressure etc..
Because of the size and width of the classical guitar neck, its just not practical to use a technique that you find in blues, rock, or rag-time styles on smaller/thinner guitar necks where the thumb is extremely exposed and used as a fretting mechanism for bass-lines while the other fingers do other chord progressions.