Yoshida has made a post stating that he is writing up a long explanation about spell interruption upon movement.

We will give you guys an update as soon as he posts and we cast our translation magic!

Quote Originally Posted by Naoki_Yoshida View Post
Hi, this is Yoshida, the Producer/Director.

I would like to comment on spells being halted by moving while casting in patch 1.20.

We are still in the process of implementing and testing this spec, but I will provide an explanation on why this is being implemented soon, so please stay patient.

When a character is moved while casting a spell, the spell is instantly halted. Once the character stops moving and casts another spell, the new spell will be cast instantly.
(Although there will be a standard MMO lag of about 0.3 seconds)

We will set up a specific key for halting spells, so we did not implement this spec as a method of halting spells.

I am preparing a long explanation regarding this, so please hang on a bit longer.