The GTK backend has settings that the WPE backend doesn't, so if you build with smooth scrolling enabled, there's no way to actually enable it on a WPE build. WPE builds should have smooth scrolling enabled if smooth scrolling is explicitly built, at least until we have an equivalent settings framework.
Created attachment 415548 [details] Patch
Thanks for the patch. If this patch contains new public API please make sure it follows the guidelines for new WebKit2 GTK+ API. See https://trac.webkit.org/wiki/WebKitGTK/AddingNewWebKit2API
To clarify that first comment, I mean it has means to change those settings that WPE doesn't. It doesn't make sense to have settings that aren't built by default that also aren't enabled when explicitly built on WPE.
This isn't correct, there is API to change this, it's just not accessible via minibrowser from the command-line like it is in GTK.