Apparently, you can't set objects directly ol n top of half blocks... and my OCD is not liking this one bit, as I've just done the floor of my entire base in half bricks... theres a one half block space between all of my odds and ends and the "floor".
I mean, I understand that pools of water shouldn't be able to be created in a half space (unless you create puddles, like... something you would never be able to turn into an infinite supply), but... come on... seriously? Floating crafting tables? Sunken beds? (The sunken bed is actually kind of cool... gives it a very campy feel).
Currently the only thing that can be placed directly on a bottom slab (or directly beneath a top slab) is another slab of the same type…
Allowing full blocks to simultaneously occupy the top and bottom half of adjacent cubes would open a lot of new possibilities, but would also require a major rework of the game (and might be a start down a slippery slope with regards to ever finer block placement mechanics).
For these reasons, I doubt any fix to this is in the offing…
…and – as you note – the current system can sometimes be used to good effect. [Although a bed requires at least one solid block with a full top surface with 2 transparent blocks (typically air) above it among the 10 blocks around the bed for the bed to allow respawns.]
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WARNING: I have an extemely "grindy" playstyle; YMMV — if this doesn't seem fun to you, mine what you can from it & bin the rest.
Maybe it was just a tease for pillagers or pillager beasts, but I believe in one of the teaser photos posted for a 1.14 snapshot included a dark grey block that was highly-obscured on the right edge of the picture. The eagle-eyed people obsessing over these teasers were thinking it was some sort of block that was placed half a block offset from the grid, meaning (according to them) we were potentially getting a half-block or quarter-block grid but still only be allowed full-block/half-block blocks to place.
Not enough information was available to speculate on whether two blocks of different materials could occupy the same spaces as two slabs of the same material, though.
Although a bed requires at least one solid block with a full top surface with 2 transparent blocks (typically air) above it among the 10 blocks around the bed for the bed to allow respawns.
Transparent in this case means you can physically walk through it. Way long ago, Etho demonstrated a trick with beds surrounded with glass that did something nifty (I forget what, though). That broke, however, and glass around beds now invalidates the bed spawn.