Created attachment 8173 Cropped icon The first horizontal pixel line and the last are not visible. The icon is only truncated when the height of the panel bar has an even number. For uneven numbers the icon is fully visible. I am using Xubuntu 18.10 64 bit.
To be able to test this the icons have to be big enough and touch the "edge" of the image file. Lots of icons for example have transparent space in the first and last horizontal pixel line.
You can test this by installing the audacious media player. The icon of the player in the xfce4 panel is cropped.
This problem is fixed in git master already: https://git.xfce.org/xfce/xfce4-panel/commit/?id=3abbe78bfa3c643f7d31338e97682d8490a09a59
Not sure about this patch but xfce panel version 4.13.4 works fine for me.
The package in Xubuntu 19.04 includes the patch. https://launchpad.net/ubuntu/+source/xfce4-panel/4.13.4-1ubuntu1