When you use the drawSelection() extension, it hides the current selection through:

& .cm-line ::selection {
    background-color: transparent !important;
}

However, we were styling selections using:

&.cm-focused .cm-line ::selection {
    background-color: <some background color> !important;
}

Since our style is more selective, it takes precedence over the original style that was supposed to hide the current selection. As a result, if you use drawSelection(), the selection would be highlighted twice, once through .cm-selectionBackground and once through .cm-line::selection.