The bug

When you move an item from your inventory in the trading menu of a villager to a spot where an item is displayed:

• The count of the displayed item gets displayed in front of your item
• The count of your item get displayed behind the displayed item

The reason

The following is based on a decompiled version of Minecraft 1.9 using MCP 9.24 beta.

The reason for this is that the method net.minecraft.client.renderer.RenderItem.renderItemOverlayIntoGUI(FontRenderer, ItemStack, int, int, String) always disables depth before drawing the item count. This way it is always drawn "on top" of everything. The problem is that GUIs that extend GuiContainer always draw after the container itself is drawn. As the GuiContainer class also draws the item picked up with the mouse, the item count drawn by the villager GUI is on top of it.
This could (and maybe should) be solved by having the method net.minecraft.client.renderer.RenderItem.renderItemOverlayIntoGUI(FontRenderer, ItemStack, int, int, String) use like it is done for drawing the items use a z-level as well to determine the depth.