From 67e1125b4c5471f70a834067968222c4b88dd2e5 Mon Sep 17 00:00:00 2001
From: Sam Atkins <atkinssj@serenityos.org>
Date: Fri, 3 Dec 2021 12:52:14 +0000
Subject: [PATCH] LibWeb: Handle fallback values for CSS variables :^)

---
 Userland/Libraries/LibWeb/CSS/StyleComputer.cpp | 12 +++++++++---
 Userland/Libraries/LibWeb/CSS/StyleComputer.h   |  2 +-
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp b/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp
index 9b0e4afa33..c8ab27f583 100644
--- a/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp
+++ b/Userland/Libraries/LibWeb/CSS/StyleComputer.cpp
@@ -452,7 +452,7 @@ struct MatchingDeclarations {
     Vector<MatchingRule> author_rules;
 };
 
-bool StyleComputer::expand_unresolved_values(DOM::Element& element, Vector<StyleComponentValueRule> const& source, Vector<StyleComponentValueRule>& dest) const
+bool StyleComputer::expand_unresolved_values(DOM::Element& element, Vector<StyleComponentValueRule> const& source, Vector<StyleComponentValueRule>& dest, size_t source_start_index) const
 {
     // FIXME: Do this better!
     // We build a copy of the tree of StyleComponentValueRules, with all var()s replaced with their contents.
@@ -468,7 +468,8 @@ bool StyleComputer::expand_unresolved_values(DOM::Element& element, Vector<Style
         return nullptr;
     };
 
-    for (auto& value : source) {
+    for (size_t source_index = source_start_index; source_index < source.size(); source_index++) {
+        auto& value = source[source_index];
         if (value.is_function()) {
             if (value.function().name().equals_ignoring_case("var"sv)) {
                 auto& var_contents = value.function().values();
@@ -489,7 +490,12 @@ bool StyleComputer::expand_unresolved_values(DOM::Element& element, Vector<Style
                     continue;
                 }
 
-                // TODO: Handle fallback value
+                // Use the provided fallback value, if any.
+                if (var_contents.size() > 2 && var_contents[1].is(Token::Type::Comma)) {
+                    if (!expand_unresolved_values(element, var_contents, dest, 2))
+                        return false;
+                    continue;
+                }
             }
 
             auto& source_function = value.function();
diff --git a/Userland/Libraries/LibWeb/CSS/StyleComputer.h b/Userland/Libraries/LibWeb/CSS/StyleComputer.h
index 3df5eff4d1..e3a6a1b106 100644
--- a/Userland/Libraries/LibWeb/CSS/StyleComputer.h
+++ b/Userland/Libraries/LibWeb/CSS/StyleComputer.h
@@ -62,7 +62,7 @@ private:
     void compute_defaulted_property_value(StyleProperties&, DOM::Element const*, CSS::PropertyID) const;
 
     RefPtr<StyleValue> resolve_unresolved_style_value(DOM::Element&, PropertyID, UnresolvedStyleValue const&) const;
-    bool expand_unresolved_values(DOM::Element&, Vector<StyleComponentValueRule> const& source, Vector<StyleComponentValueRule>& dest) const;
+    bool expand_unresolved_values(DOM::Element&, Vector<StyleComponentValueRule> const& source, Vector<StyleComponentValueRule>& dest, size_t source_start_index = 0) const;
 
     template<typename Callback>
     void for_each_stylesheet(CascadeOrigin, Callback) const;