Random top border on newly created workspaces

[Chaspen]

In my stylesheet, I have the workspace module setup so that an animation plays on focus; and it works! However a weird side-effect of this is that a roughly 1-2px width border appears at the top of newly created work spaces for a few frames before going away. As far as I know, there's nothing in my CSS that could cause this, however there may be something I'm missing and would like some insight into what I can do with the relevant CSS to fix this if possible:

#workspaces button {
    background-color: transparent;
    background:transparent; 
    color: white;
    min-width: 30px;
    border-radius: 0px;
    transition: 0.2s;
}

#workspaces button.focused {
    border-top-width: 0;
    border-bottom-style: solid;
    border-bottom-width: 3px;
    border-top-width: 0px;
    padding-top: 3px;
    background: transparent;
}

#workspaces button:hover {
    /* disable annoying default hover properties */
    box-shadow: inherit;
    text-shadow: inherit;
    background: inherit;
    border: white;
    padding: 0 0px;
    /* actual shit i want */
    color: black;
    background-color: white;
}

[Chaspen]

Changing the GTK theme from the default (Adwaita) fixed this.