FIX: Replace R2 gem with rtlcss for generating RTL CSS (#19636)

We've had a couple of problems with the R2 gem where it generated a broken RTL CSS bundle that caused a badly broken layout when Discourse is used in an RTL language, see a3ce93b and 5926386. For this reason, we're replacing R2 with `rtlcss` that can handle modern CSS features better than R2 does.

`rltcss` is written in JS and available as an npm package. Calling the `rltcss` from rubyland is done via the `rtlcss_wrapper` gem which contains a distributable copy of the `rtlcss` package and loads/calls it with Mini Racer. See https://github.com/discourse/rtlcss_wrapper for more details.

Internal topic: t/76263.

Gemfile

@@ -105,7 +105,7 @@ gem "pg"
 gem "mini_sql"
 gem "pry-rails", require: false
 gem "pry-byebug", require: false
-gem "r2", require: false
+gem "rtlcss_wrapper", require: false
 gem "rake"
 
 gem "thor", require: false

Gemfile.lock

@@ -315,7 +315,6 @@ GEM
     public_suffix (5.0.1)
     puma (6.0.2)
       nio4r (~> 2.0)
-    r2 (0.2.7)
     racc (1.6.2)
     rack (2.2.6.2)
     rack-mini-profiler (3.0.0)
@@ -398,6 +397,8 @@ GEM
       json-schema (>= 2.2, < 4.0)
       railties (>= 3.1, < 7.1)
       rspec-core (>= 2.14)
+    rtlcss_wrapper (0.1.0)
+      mini_racer (~> 0.6.3)
     rubocop (1.44.1)
       json (~> 2.3)
       parallel (~> 1.10)
@@ -600,7 +601,6 @@ DEPENDENCIES
   pry-byebug
   pry-rails
   puma
-  r2
   rack
   rack-mini-profiler
   rack-protection
@@ -621,6 +621,7 @@ DEPENDENCIES
   rspec-rails
   rss
   rswag-specs
+  rtlcss_wrapper
   rubocop-discourse
   ruby-prof
   ruby-readability

app/assets/stylesheets/common/base/sidebar-more-section-links.scss

@@ -44,27 +44,7 @@
     margin: 0 calc(var(--d-sidebar-row-horizontal-padding) * 2 / 3);
 
     .sidebar-row {
-      // the multiplication by 1 here is a workaround for a bug in the R2 gem
-      // that we use to generate RTL CSS.
-      // the gem generates RTL CSS by converting anything left to right and
-      // vice versa. for example, a `padding-right: 1px;` property becomes
-      // `padding-left: 1px;` when it goes through the gem.
-      // the gem also handles the `padding` property (and similar properties)
-      // when it's in the 4-sides form, e.g. `padding: 1px 2px 3px 4px;` which
-      // gets converted to `padding: 1px 4px 3px 2px;`.
-      // however, the problem is that the gem detects 4-sides properties pretty
-      // naively - it splits the property value on /\s+/ and if it has 4 parts,
-      // it swaps the second and fourth parts.
-      // if you remove the by 1 multiplication in our rule below, we end up
-      // with a value that can be split into 4 parts and that causes the R2 gem
-      // to convert the rule to this:
-      //   padding: 0.33rem 3) / calc(var(--d-sidebar-row-horizontal-padding);
-      // which is clearly invalid and breaks all the rules that come after this
-      // one in the application CSS bundle.
-      // in the long term we should probably find (or write ourselves)
-      // something that's smarter than R2, but for now let's workaround the bug
-      // in R2.
-      padding: 0.33rem calc(1 * var(--d-sidebar-row-horizontal-padding) / 3);
+      padding: 0.33rem calc(var(--d-sidebar-row-horizontal-padding) / 3);
     }
   }
 

app/assets/stylesheets/common/components/hashtag.scss

@@ -55,7 +55,7 @@ a.hashtag-cooked {
       background: linear-gradient(
         to bottom,
         rgba(var(--secondary-rgb), 0),
-        rgba(var(--secondary-rgb), 1)
+        rgba(var(--secondary-rgb), 100%)
       );
     }
   }

lib/stylesheet/compiler.rb

@@ -65,8 +65,8 @@ module Stylesheet
       result = engine.render
 
       if options[:rtl]
-        require "r2"
-        [R2.r2(result), nil]
+        require "rtlcss_wrapper"
+        [RtlcssWrapper.flip_css(result), nil]
       else
         source_map = engine.source_map
         source_map.force_encoding("UTF-8")

spec/lib/stylesheet/compiler_spec.rb

@@ -214,4 +214,11 @@ RSpec.describe Stylesheet::Compiler do
       expect(refs).to eq([])
     end
   end
+
+  describe ".compile" do
+    it "produces RTL CSS when rtl option is given" do
+      css, _ = Stylesheet::Compiler.compile("a{right:1px}", "test.scss", rtl: true)
+      expect(css).to eq("a{left:1px}")
+    end
+  end
 end