moving cards in disabled editor can cause `editor.range` to go out of sync

[bantic]

In a disabled editor, the _readRangeFromDOM method no-ops. This can cause editor.range to stay cached on an old state. If that range references sections that have since been programmatically deleted from the post, unexpected errors can happen, for example when snapshotting the range (to add to the undo stack), the range.head.section is expected to have a post property, but if that section has been removed it no longer will. This causes an error in the position#leafSectionIndex code.