It's currently returning always 0 for WebKitGTK+. https://www.w3.org/TR/uievents/#ref-for-dom-mouseevent-buttons-1
Created attachment 338644 [details] Patch
Comment on attachment 338644 [details] Patch View in context: https://bugs.webkit.org/attachment.cgi?id=338644&action=review > Source/WebKit/Shared/gtk/WebEventFactory.cpp:154 > + unsigned modifier = 1 << (8 + event->button.button - 1); I've seen this calculation about five times in this patch; worth writing a small helper function?
Committed r230988: <https://trac.webkit.org/changeset/230988>