[webkit-gtk] How to enable SelectionPaintingWithoutSelectionGaps?

[Artyom]

I want to get rid of "blocky" selection in webkit (see this bug:
https://bugs.webkit.org/show_bug.cgi?id=156900). Apparently, to do this
I should enable SelectionPaintingWithoutSelectionGaps. I found it in the
source (in WKPreferences), but I can't understand how to enable it from
WebKitGTK. Is it even possible, or are some settings from WKPreferences
not exposed by WebKitGTK?