Prevent fallback backdrop filters on root.

Much like https://searchfox.org/mozilla-central/rev/4519912ada795963299e4ba84c324adba762eff3/layout/painting/nsDisplayList.cpp#8418

Differential Revision: https://phabricator.services.mozilla.com/D149236

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1770063
gecko-commit: 49223a431cd47064e5d1c045ff108f55a806bf05
gecko-reviewers: gfx-reviewers, gw
diff --git a/css/filter-effects/backdrop-filter-feimage-crash.html b/css/filter-effects/backdrop-filter-feimage-crash.html
new file mode 100644
index 0000000..762452e
--- /dev/null
+++ b/css/filter-effects/backdrop-filter-feimage-crash.html
@@ -0,0 +1,11 @@
+<!doctype html>
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1770063">
+<style>
+* {
+  backdrop-filter: url(#a);
+  filter: saturate(60%);
+}
+</style>
+<svg>
+<filter id='a'>
+<feImage>