Clamp insert index

Because we don't fully process CSS, indexes that code expect might be out of
bounds. The specific case comes from https://github.com/lightpanda-io/browser/issues/2214
which uses fullcalender library. This library uses a @font-face which we do not
process and thus end up with incorrect indexes. As a quick workaround aligned
with previous fixes for these types of cases (and what the original issue
recommended), we simply clamp the index.

src/browser/tests/css/stylesheet.html

@@ -498,6 +498,28 @@
 }
 </script>
 
+<script id="CSSStyleSheet_insertRule_out_of_range_index">
+{
+    // Our lack of fully processing sources (e.g. @import, @font-face) means that
+    // our indexes are sometimes not what libraries expect. As a workaround we
+    // clamp the index to the end of the rule list. See #2214.
+    const style = document.createElement('style');
+    document.head.appendChild(style);
+    const sheet = style.sheet;
+
+    const returned = sheet.insertRule('.out-of-range { color: red; }', 99);
+    testing.expectEqual(0, returned);
+    testing.expectEqual(1, sheet.cssRules.length);
+    testing.expectEqual('.out-of-range', sheet.cssRules[0].selectorText);
+
+    // And at the tail of a non-empty sheet
+    const tail = sheet.insertRule('.tail { color: blue; }', 99);
+    testing.expectEqual(1, tail);
+    testing.expectEqual(2, sheet.cssRules.length);
+    testing.expectEqual('.tail', sheet.cssRules[1].selectorText);
+}
+</script>
+
 <script id="CSSStyleSheet_replaceSync">
 {
     const sheet = new CSSStyleSheet();

src/browser/webapi/css/CSSStyleSheet.zig

@@ -79,7 +79,7 @@ pub fn getOwnerRule(self: *const CSSStyleSheet) ?*CSSRule {
 }
 
 pub fn insertRule(self: *CSSStyleSheet, rule: []const u8, maybe_index: ?u32, frame: *Frame) !u32 {
-    const index = maybe_index orelse 0;
+    const requested_index = maybe_index orelse 0;
     var it = Parser.parseStylesheet(rule);
     const parsed_rule = it.next() orelse {
         if (it.has_skipped_at_rule) {
@@ -88,7 +88,7 @@ pub fn insertRule(self: *CSSStyleSheet, rule: []const u8, maybe_index: ?u32, fra
             // CSS parser. To prevent JS apps (like Expo/Reanimated) from crashing
             // during initialization, we simulate a successful insertion by returning
             // the requested index.
-            return index;
+            return requested_index;
         }
         return error.SyntaxError;
     };
@@ -103,6 +103,16 @@ pub fn insertRule(self: *CSSStyleSheet, rule: []const u8, maybe_index: ?u32, fra
     try style.setCssText(parsed_rule.block, frame);
 
     const rules = try self.getCssRules(frame);
+
+    // Per spec, an index > rules.length should throw IndexSizeError. But because
+    // we don't process @import and @font-face, indexes that code hard-codes can
+    // be off. As a workaround, we clamp to the tail.
+    // See #2214 (and the sibling #1970 / #1972 tolerance for at-rules).
+    const length = rules.length();
+    const index = if (requested_index > length) length else requested_index;
+    if (index != requested_index) {
+        log.debug(.not_implemented, "insertRule clamped index", .{});
+    }
     try rules.insert(index, style_rule._proto, frame);
 
     // Notify StyleManager that rules have changed