I've been using this program for a while (it works for far more than just Mabinogi)
Source GithubSource SteamPageWhat this program does is it can automatically maximize or borderless fullscreen games that are in windowed mode. For Mabinogi you can do the following.
Select Mabinogi [Client] and click the > arrow (this enables automatic processing and saved settings for this process
This will default to borderless fullscreen. If you'd prefer to have auto-maximizing instead, right click Mabinogi [Max] on the right side, click No Size change, then right click again and click "Set Window Size" Then click no, 0, 0, your screen width, and then your screen height (minus the taskbar at the bottom)
If done correctly, the right side should say "Mabinogi [0,0- Xres,Yres]". For example, a 1920x1080 display would be 0,0-1920,1050.
Notes: This can be used as an alternative to Abyss' function to handle window position and auto-maximizing, however this *will* hide the titlebar. You can reveal it by selecting Mabinogi[Client] and then clicking the inward facing arrows. (outward facing will force-process the settings for that application, if automatic doesn't)