There are several factors that can lead to this issue appearing, so the first thing you need to do is to understand the problem. If you get a thick black border around the Ulauncher window, it's probably because your window manager does not support compositing, and the border is actually how your window manager renders the shadow around the Ulauncher window.

While if you get a thin border and/or a title bar, this is because your window manager does not respect WM hints to hide the title bar.

Disabling shadows for compositing window manager

If you don't use a compositing window manager, then Ulauncher's shadows will look like big black box or similar on Ulauncher 5 and older. To avoid this you need to start ulauncher with a command line flag: ulauncher --no-window-shadow.

Ulauncher version 6 (beta) has removed this flag in favor of trying to detect and disable shadows when the compositor doesn't support them. This behavior has been confirmed to work on i3.

Window manager specific workarounds to hide borders

Window managers and DEs can add their own borders by default that you have to override. See examples how to do this for specific DEs:

KDE Plasma

Most likely it will just work if you use Ulauncher v6 and have gir1.2-gtklayershell-0.1 installed. If this does not work, see dedicated page for details.

Sway

You have to tell sway not to put borders on the ulauncher window. Example sway config:

exec GDK_BACKEND=wayland ulauncher --hide-window
for_window [app_id="ulauncher"] floating enable, border none

Credit to comment by evilhamsterman

Picom

blur-background-exclude = [
  # others
  "class_g  = 'Ulauncher'"
]

Credit to comment by twnaing

BSPWM

bspc rule -a Ulauncher state=floating border=off

Credit to this Reddit post

Compton

compton --shadow-exclude 'class_g = Ulauncher'

See this Compton issue for more details

Credit to comment by subdavis