Fenced blockquotes to not change source line pos

Replaces blockquote fences with \n,
keeping the line numbering intact.

changelogs/unreleased/58717-checkbox-cannot-be-checked-if-a-blockquote-is-above.yml

+---
+title: Allow task lists that follow a blockquote to work correctly
+merge_request: 26937
+author:
+type: fixed

lib/banzai/filter/blockquote_fence_filter.rb

@@ -42,7 +42,9 @@ module Banzai
       def call
         @text.gsub(REGEX) do
           if $~[:quote]
-            $~[:quote].gsub(/^/, "> ").gsub(/^> $/, ">")
+            # keep the same number of source lines/positions by replacing the
+            # fence lines with newlines
+            "\n" + $~[:quote].gsub(/^/, "> ").gsub(/^> $/, ">") + "\n"
           else
             $~[0]
           end

spec/fixtures/blockquote_fence_after.md

@@ -18,10 +18,13 @@ Double `>>>` inside code block:
 
 Blockquote outside code block:
 
+
 > Quote
 
+
 Code block inside blockquote:
 
+
 > Quote
 >
 > ```
@@ -30,8 +33,10 @@ Code block inside blockquote:
 >
 > Quote
 
+
 Single `>>>` inside code block inside blockquote:
 
+
 > Quote
 >
 > ```
@@ -42,8 +47,10 @@ Single `>>>` inside code block inside blockquote:
 >
 > Quote
 
+
 Double `>>>` inside code block inside blockquote:
 
+
 > Quote
 >
 > ```
@@ -56,6 +63,7 @@ Double `>>>` inside code block inside blockquote:
 >
 > Quote
 
+
 Single `>>>` inside HTML:
 
 <pre>
@@ -76,10 +84,13 @@ Double `>>>` inside HTML:
 
 Blockquote outside HTML:
 
+
 > Quote
 
+
 HTML inside blockquote:
 
+
 > Quote
 >
 > <pre>
@@ -88,8 +99,10 @@ HTML inside blockquote:
 >
 > Quote
 
+
 Single `>>>` inside HTML inside blockquote:
 
+
 > Quote
 >
 > <pre>
@@ -100,8 +113,10 @@ Single `>>>` inside HTML inside blockquote:
 >
 > Quote
 
+
 Double `>>>` inside HTML inside blockquote:
 
+
 > Quote
 >
 > <pre>
@@ -113,3 +128,4 @@ Double `>>>` inside HTML inside blockquote:
 > </pre>
 >
 > Quote
+

spec/lib/banzai/filter/blockquote_fence_filter_spec.rb

@@ -13,6 +13,6 @@ describe Banzai::Filter::BlockquoteFenceFilter do
   end
 
   it 'allows trailing whitespace on blockquote fence lines' do
-    expect(filter(">>> \ntest\n>>> ")).to eq("> test")
+    expect(filter(">>> \ntest\n>>> ")).to eq("\n> test\n")
   end
 end

spec/services/issues/build_service_spec.rb

@@ -58,8 +58,10 @@ describe Issues::BuildService do
                     "> That has a quote\n"\
                     ">>>\n"
         note_result = "    > This is a string\n"\
+                      "    > \n"\
                       "    > > with a blockquote\n"\
-                      "    > > > That has a quote\n"
+                      "    > > > That has a quote\n"\
+                      "    > \n"
         discussion = create(:diff_note_on_merge_request, note: note_text).to_discussion
         expect(service.item_for_discussion(discussion)).to include(note_result)
       end

spec/services/task_list_toggle_service_spec.rb

@@ -113,4 +113,25 @@ describe TaskListToggleService do
 
     expect(toggler.execute).to be_falsey
   end
+
+  it 'properly handles a GitLab blockquote' do
+    markdown =
+      <<-EOT.strip_heredoc
+      >>>
+      gitlab blockquote
+      >>>
+
+      * [ ] Task 1
+      * [x] Task 2
+    EOT
+
+    markdown_html = Banzai::Pipeline::FullPipeline.call(markdown, project: nil)[:output].to_html
+    toggler = described_class.new(markdown, markdown_html,
+                                  toggle_as_checked: true,
+                                  line_source: '* [ ] Task 1', line_number: 5)
+
+    expect(toggler.execute).to be_truthy
+    expect(toggler.updated_markdown.lines[4]).to eq "* [x] Task 1\n"
+    expect(toggler.updated_markdown_html).to include('disabled checked> Task 1')
+  end
 end