The user-defined Desktop Grid Border Width parameter was removed in the QML rewrite, this MR originally restored it to the kcm_kwin_effects module, and integrated the value into src/effects/desktopgrid/qml/main.qml. This mostly reused the old code and parameters, but considers new structure in src/effects/desktopgrid/desktopgrideffect.cpp following the rewrite.
However, considering my use case where I simply set the integer to 0 to disable borders entirely, and that the previous solution was rather complex in config (0 to 100 pixels vs presets or a toggle), I've reworked the request into a single toggle to disable borders within the Grid effect.
