Irregularities with Gridstack and moving of blocks

[Kristina Hoeppner]

Tested in Mahara 22.10 and main. When you add blocks to an already populated page, there was an issue with existing blocks not always moving as required but having a sort of invisible placeholder set although there shouldn't be any.

See the screencast for the problem description. Once we've identified the cause we can write up the test scenario for what we can fix.