When a filesystem is resized by offline growing and the partiton is enlargened, the GVFs automount is executed at the same time as the filesystem should be resized. This is due to the fact that the partition and its filesystem is newly discovered in UDisks.
If the mounting is faster this leads to a failing resize operation.

As this would require changes in UDisks logic of processing udev events the best way to solve it, I think, is to just leave this corner case away and do the same thing as in the online grow procedure:

Let the filesystem mounted while enlargening the partition. This causes no problems because the kernel gets notified about the changed partition size. Because the filesystem is mounted there is no automount due to discovering the partition anew.
After the partition is grown the filesystem needs to be unmounted to be shrunk and in the following operations trigger no new discovery which would lead to automounts.

The patch takes also care of the corner case when the user unmounts the filesystem while the dialog is open and will not remount it if it would be unmounted directly for shrinking.