From 8c1ea0908af0371f8818259546cb3e3d68e55559 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Emilio=20Cobos=20=C3=81lvarez?= <emilio@crisal.io>
Date: Thu, 8 Jan 2026 07:50:17 +0000
Subject: [PATCH] Don't unconditionally fall back to single inserts for
 ::first-letter.

This fixes the regression making us go through the same codepath as
before, but I think this assertion is fishy, see details on the bug.

Differential Revision: https://phabricator.services.mozilla.com/D278121

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=2008625
gecko-commit: 7c7cb9e2ce5c59179d9a0eed7f1dd31472c8a137
gecko-reviewers: TYLin, layout-reviewers
---
 .../first-letter-column-span-fieldset.html       | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)
 create mode 100644 css/css-multicol/crashtests/first-letter-column-span-fieldset.html

diff --git a/css/css-multicol/crashtests/first-letter-column-span-fieldset.html b/css/css-multicol/crashtests/first-letter-column-span-fieldset.html
new file mode 100644
index 00000000000000..71815a18f27438
--- /dev/null
+++ b/css/css-multicol/crashtests/first-letter-column-span-fieldset.html
@@ -0,0 +1,16 @@
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=2008625">
+<style>
+*:last-of-type {
+  columns: 474px 1;
+}
+*:valid {
+  column-span: all;
+}
+*::first-letter { }
+</style>
+<data>
+<form></form>
+</data>
+<u>
+<fieldset></fieldset>
+</u>