Hi, this is a heads-up that the webkit_settings_set_enable_frame_flattening() and the WebKitSettings:enable-frame-flattening property will break in WebKitGTK 2.40. You'll need to figure out how to stop using them soon. If your app breaks with this setting disabled, then you'll want to ensure updated versions of your application reach environments that you care about prior to the release of WebKitGTK 2.40.
For more information: https://bugs.webkit.org/show_bug.cgi?id=242883
Sorry for the trouble. :/