Running from git tree on Arch. The default new panel height is 48. In 4.12, when creating a new panel, the new panel would display with the correct height. On the current git version, the panel height appears to be around 30 even though the value on the properties dialog says 48. Any change to the size value in this dialog automatically re-adjusts the size correctly. One of the issues with this inconsistency is that when adding some plugins, example Window Buttons, renders them invisible until the size discrepancy is fixed.
Created attachment 7740 Fix initial panel size A potential patch to set the initial panel size to the default 48.
Tony Paulic referenced this bugreport in commit 77e579478473d923fcf6c05a50a403f046dcea99 Fix default size of new panels (Bug #14417) https://git.xfce.org/xfce/xfce4-panel/commit?id=77e579478473d923fcf6c05a50a403f046dcea99
Thanks for the patch! (Btw, setting the panel's default size to 48 explicitly fixed another bug with launchers being wrongly sized.)