In current Rawhide, after an install from the KDE live image, the desktop selector drop-down box on the left hand side of SDDM seems blank. It's just all grey, with the drop-down arrow on the end. There is no text - it should say 'Plasma'.

If you click on the arrow, you get a black box with a grey border and no visible text. If you mouse over this and then mouse away, it becomes a grey box with 'Plasma' visible in black text, and the cursor (bizarrely) becomes the X fallback 'cross' cursor. If you hover the mouse over the box, it turns back into the black box, and the cursor fixes itself; as soon as you move the mouse away from the box again, it goes back to being the grey box with 'Plasma' visible, and the cursor breaks again.