You can't place a slab underneath an already placed slab
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I too can confirm this issue exists. To add further detail to it...
Prior to version 1.9, you could place slabs by right-clicking in the empty space on one of the containing walls as pictured in the screenshot. This is no longer possible, and you must always right-click in a specific place.
[media]If you are trying to fill a space on top of a bottom-half slab, you must right-click on top of the half slab below. Conversely, if you are trying to fill the gap below a top-half slab, then you must click on the bottom of the slab that sits above the gap. This makes it impossible to fill a bottom slab gap in if you have a floor that you don't wish to destroy. It would be great if we could go back to the old logistics for placing slabs!
I too can confirm this issue exists. To add further detail to it...
Prior to version 1.9, you could place slabs by right-clicking in the empty space on one of the containing walls as pictured in the screenshot. This is no longer possible, and you must always right-click in a specific place.
[media]If you are trying to fill a space on top of a bottom-half slab, you must right-click on top of the half slab below. Conversely, if you are trying to fill the gap below a top-half slab, then you must click on the bottom of the slab that sits above the gap. This makes it impossible to fill a bottom slab gap in if you have a floor that you don't wish to destroy. It would be great if we could go back to the old logistics for placing slabs!