From c02163c31fe8301b35419c6d7fc75b1e30f58c80 Mon Sep 17 00:00:00 2001
From: MacDue <macdue@dueutil.tech>
Date: Tue, 8 Nov 2022 20:50:00 +0100
Subject: [PATCH] LibWeb: Allow optional values to be missing when parsing
 `<position>`s

---
 .../Libraries/LibWeb/CSS/Parser/Parser.cpp    | 50 +++++++++++--------
 Userland/Libraries/LibWeb/CSS/Parser/Parser.h |  2 +-
 2 files changed, 30 insertions(+), 22 deletions(-)

diff --git a/Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp b/Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp
index bb8afd11d7..a265c6718f 100644
--- a/Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp
+++ b/Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp
@@ -2644,7 +2644,7 @@ RefPtr<StyleValue> Parser::parse_conic_gradient_function(ComponentValue const& c
     return ConicGradientStyleValue::create(from_angle, at_position, move(*color_stops), repeating_gradient);
 }
 
-Optional<PositionValue> Parser::parse_position(TokenStream<ComponentValue>& tokens)
+Optional<PositionValue> Parser::parse_position(TokenStream<ComponentValue>& tokens, PositionValue initial_value)
 {
     auto transaction = tokens.begin_transaction();
     tokens.skip_whitespace();
@@ -2700,7 +2700,7 @@ Optional<PositionValue> Parser::parse_position(TokenStream<ComponentValue>& toke
     // [ left | center | right ] || [ top | center | bottom ]
     auto alternation_1 = [&]() -> Optional<PositionValue> {
         auto transaction = tokens.begin_transaction();
-        PositionValue position {};
+        PositionValue position = initial_value;
         auto& first_token = tokens.next_token();
         if (!first_token.is(Token::Type::Ident))
             return {};
@@ -2709,15 +2709,17 @@ Optional<PositionValue> Parser::parse_position(TokenStream<ComponentValue>& toke
         auto horizontal_position = parse_horizontal_preset(ident);
         if (horizontal_position.has_value()) {
             position.horizontal_position = *horizontal_position;
+            auto transaction_optional_parse = tokens.begin_transaction();
             tokens.skip_whitespace();
             if (tokens.has_next_token()) {
                 auto& second_token = tokens.next_token();
-                if (!second_token.is(Token::Type::Ident))
-                    return {};
-                auto vertical_position = parse_vertical_preset(second_token.token().ident());
-                if (!vertical_position.has_value())
-                    return {};
-                position.vertical_position = *vertical_position;
+                if (second_token.is(Token::Type::Ident)) {
+                    auto vertical_position = parse_vertical_preset(second_token.token().ident());
+                    if (vertical_position.has_value()) {
+                        transaction_optional_parse.commit();
+                        position.vertical_position = *vertical_position;
+                    }
+                }
             }
         } else {
             // <vertical-position> <horizontal-position>?
@@ -2725,14 +2727,17 @@ Optional<PositionValue> Parser::parse_position(TokenStream<ComponentValue>& toke
             if (!vertical_position.has_value())
                 return {};
             position.vertical_position = *vertical_position;
+            auto transaction_optional_parse = tokens.begin_transaction();
+            tokens.skip_whitespace();
             if (tokens.has_next_token()) {
                 auto& second_token = tokens.next_token();
-                if (!second_token.is(Token::Type::Ident))
-                    return {};
-                auto horizontal_position = parse_horizontal_preset(second_token.token().ident());
-                if (!horizontal_position.has_value())
-                    return {};
-                position.horizontal_position = *horizontal_position;
+                if (second_token.is(Token::Type::Ident)) {
+                    auto horizontal_position = parse_horizontal_preset(second_token.token().ident());
+                    if (horizontal_position.has_value()) {
+                        transaction_optional_parse.commit();
+                        position.horizontal_position = *horizontal_position;
+                    }
+                }
             }
         }
         transaction.commit();
@@ -2743,7 +2748,7 @@ Optional<PositionValue> Parser::parse_position(TokenStream<ComponentValue>& toke
     // [ top | center | bottom | <length-percentage> ]?
     auto alternation_2 = [&]() -> Optional<PositionValue> {
         auto transaction = tokens.begin_transaction();
-        PositionValue position {};
+        PositionValue position = initial_value;
         auto& first_token = tokens.next_token();
         if (first_token.is(Token::Type::Ident)) {
             auto horizontal_position = parse_horizontal_preset(first_token.token().ident());
@@ -2756,19 +2761,22 @@ Optional<PositionValue> Parser::parse_position(TokenStream<ComponentValue>& toke
                 return {};
             position.horizontal_position = dimension->length_percentage();
         }
+        auto transaction_optional_parse = tokens.begin_transaction();
         tokens.skip_whitespace();
         if (tokens.has_next_token()) {
             auto& second_token = tokens.next_token();
             if (second_token.is(Token::Type::Ident)) {
                 auto vertical_position = parse_vertical_preset(second_token.token().ident());
-                if (!vertical_position.has_value())
-                    return {};
-                position.vertical_position = *vertical_position;
+                if (vertical_position.has_value()) {
+                    transaction_optional_parse.commit();
+                    position.vertical_position = *vertical_position;
+                }
             } else {
                 auto dimension = parse_dimension(second_token);
-                if (!dimension.has_value() || !dimension->is_length_percentage())
-                    return {};
-                position.vertical_position = dimension->length_percentage();
+                if (dimension.has_value() && dimension->is_length_percentage()) {
+                    transaction_optional_parse.commit();
+                    position.vertical_position = dimension->length_percentage();
+                }
             }
         }
         transaction.commit();
diff --git a/Userland/Libraries/LibWeb/CSS/Parser/Parser.h b/Userland/Libraries/LibWeb/CSS/Parser/Parser.h
index d224023bbc..50b765d363 100644
--- a/Userland/Libraries/LibWeb/CSS/Parser/Parser.h
+++ b/Userland/Libraries/LibWeb/CSS/Parser/Parser.h
@@ -255,7 +255,7 @@ private:
     Optional<GridMinMax> parse_min_max(Vector<ComponentValue> const&);
     Optional<GridRepeat> parse_repeat(Vector<ComponentValue> const&);
     Optional<ExplicitGridTrack> parse_track_sizing_function(ComponentValue const&);
-    Optional<PositionValue> parse_position(TokenStream<ComponentValue>&);
+    Optional<PositionValue> parse_position(TokenStream<ComponentValue>&, PositionValue initial_value = PositionValue::center());
 
     enum class AllowedDataUrlType {
         None,