Thanks for the explanation, that helped me to narrow down the behaviour of ST and scrollbars.
Despite disabling custom themes the scrollbars remained and there was no effect to no avail.
One might also want to check the Accessibility Settings in the Systempreferences on macos, if there is anything, but concerning scrollbars there are no such settings.
Also despite the claim that the scrollbars on macos would be overlay by default it turned out, you can activate them to permanently to show (which I obviously did) in macos, which on default setting only means that they are displayed when the content of the window has more items than the window would show. Again this setting has no effect on ST. The second setting for scrollbars in the Systempreferences on macos however brings the result that scrollbars are infact overlay and only visible when scrolling.
If and only if you have this setting in the Systempreferences on macos you will have the overlay scrollbars in ST too.
For a different behaviour on macos and ST you want to check the Preferences of ST.
There you will find:
// Valid values are "system", "enabled" and "disabled"
"overlay_scroll_bars": "system"
Just copy this portion to your User Preferences and change the setting like so:
// Valid values are "system", "enabled" and "disabled"
"overlay_scroll_bars": "enabled"
As shown in the example you want to “enable” this setting in ST when you have another behaviour selected in your Systempreferences on macos and still want to get rid of the permanent visible scrollbars for good.