The problem is that I am able to select a window via Window Buttons in xfce4-panel only when the mouse cursor is not touching screen border. When the cursor is touching screen border, the window button is not highlited as it would be on hover and window border isn't drawn if it is enabled. This problem occurs both when the panel is at bottom and at the top of the screen. There is no similar problem with any other xfce4-panel plugin such as Whisker-menu, application launchers, or keyboard layout chooser. This started happening after upgrading to Xubuntu 18.10 from 18.04
There are unreleased fixes for this: https://git.xfce.org/xfce/xfce4-panel/commit/?id=e56e8699e271cea209f5b283421952d9035ad2b5 https://github.com/shimmerproject/Greybird/commit/0904216995f1108f517b98432000fb70a021932d
*** This bug has been marked as a duplicate of bug 14729 ***
The full path for me was /usr/share/themes/Greybird/gtk-3.0/gtk-contained.css in Ubuntu 18.10.