It's possible to place slabs inside yourself.
Steps:
Place a slab on the ground.
Step on it.
Place a block diagonal to it.
Move to a position where you stand partially on the slab, partially in the air next to the block.
Place the slab at the top half of the block you stand in by clicking at the top half of the block.
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Relates to MC-100621

I think it's not just related, but duplicate, because it handles placement from all blocks that are lower than one block.