resolves #1407 add an unbreable option on open blocks

The content of an unbreakable open block will be kept together on the same page (if it fits on one page).

lib/asciidoctor/pdf/converter.rb

@@ -931,9 +931,14 @@ def convert_open node
       # TODO process abstract child even when partintro has multiple blocks
       convert_abstract node.blocks[0]
     else
-      add_dest_for_block node if node.id
-      layout_caption node.title if node.title?
-      convert_content_for_block node
+      doc = node.document
+      keep_together_if node.option? 'unbreakable' do
+        push_scratch doc if scratch?
+        add_dest_for_block node if node.id
+        layout_caption node.title if node.title?
+        convert_content_for_block node
+        pop_scratch doc if scratch?
+      end
     end
   end
 

spec/example_spec.rb

@@ -1,6 +1,30 @@
 require_relative 'spec_helper'
 
 describe 'Asciidoctor::PDF::Converter - Example' do
+  it 'should keep block together when block has the unbreakable option' do
+    to_file = to_pdf_file <<~EOS, 'example-unbreakable-option.pdf'
+    Make it rain.footnote:[money]
+
+    #{(['filler'] * 21).join %(\n\n)}
+    [%unbreakable]
+    --
+    To install Antora, open a terminal and type:
+    
+     $ npm i -g @antora/cli@2.2 @antora/site-generator-default@2.2
+    
+    IMPORTANT: The `@` at the beginning of the package name is important.
+    It tells `npm` that the `cli` package is located in the `antora` group.
+    If you omit this character, `npm` will assume the package name is the name of a git repository on GitHub.
+    The second `@` offsets the requested version number.footnote:[Clarification about this statement.]
+    Only the major and minor segments are specified to ensure you receive the latest patch update.
+    --
+
+    Make it snow.footnote:[dollar bills]
+    EOS
+
+    (expect to_file).to visually_match 'example-unbreakable-option.pdf'
+  end
+
   it 'should keep block together if it can fit on one page' do
     pdf = to_pdf <<~EOS, analyze: true
     #{(['filler'] * 15).join %(\n\n)}