Created attachment 471731 [details] Test case reduction See the attached test case. When the header element has only one child with a backdrop-filter applied to it, everything works fine. When the header element has two children, with the backdrop-filter applied to the younger child (which comes first in the stacking order), the resulting backdrop seems to include the older child as well. This can be seen by the light text shadow (caused by the blur and the color inversion), which should not be there. The test case is rendered correctly by Firefox and Chromium (without the light text shadow). WebKit version: WebKitGTK 2.44.2 (Epiphany Browser on Linux)
Created attachment 471732 [details] Test case rendered by WebKit
Created attachment 471733 [details] Test case rendered by Firefox and Chromium
This doesn't appear to reproduce in Safari, so seems Gtk-only.
Thanks for testing this! I updated the report accordingly.