You can't see the mouse, but if the mouse is any further left and is under the top Icon bar, then the item that is placable is not shown.
(mouse just outside the top toolbutton bar)
(mouse just slightly left of there, chest is not shown)
You can still click and place the item, there's just no indication where you're placing it beforehand.
[Rseding91] [for 0.16] Map Editor - Placable Item hidden in hand
Re: [Twinsen] [for 0.16] Map Editor - Placable Item hidden in hand
Thanks for the report. This is now fixed for the next version of 0.15.
If you want to get ahold of me I'm almost always on Discord.