Padding is read from wrong box when creating new page

[mojavelinux]

When creating a new page, the padding is read from the margin box instead of the current bounding box. This can cause the indent function to effectively remove the padding twice, leading to a left margin that migrates further and further to the left.

I reported a similar problem in prawn-table. See prawnpdf/prawn-table#114

What it comes down to is that the bounding_box and indent functions don't play well together. Here's a script that reproduces the problem:

Prawn::Document.generate 'test.pdf' do
  initial_absolute_left = bounds.absolute_left
  text 'paragraph'
  bounding_box [0, cursor], width: bounds.width do
    text 'paragraph in bounding box'
    bounds.move_past_bottom
    text 'paragraph in bounding box'
  end
  indent 20 do
    bounds.move_past_bottom
  end
  if bounds.absolute_left != initial_absolute_left
    warn 'bounds were not properly restored after call to indent'
  end
  text 'paragraph'
end

The problematic logic is here:

prawn/lib/prawn/document.rb

Lines 723 to 731 in c5842a2

	# This check maintains indentation settings across page breaks
	if old_margin_box
	@margin_box.add_left_padding(old_margin_box.total_left_padding)
	@margin_box.add_right_padding(old_margin_box.total_right_padding)
	end
	# we must update bounding box if not flowing from the previous page
	#
	@bounding_box = @margin_box unless @bounding_box&.parent

I believe the following logic will resolve the problem:

# update bounding box if not flowing from the previous page
unless @bounding_box && @bounding_box.parent
  old_margin_box = @bounding_box
  @bounding_box = @margin_box
end

# maintains indentation settings across page breaks
if old_margin_box
  @margin_box.add_left_padding(old_margin_box.total_left_padding)
  @margin_box.add_right_padding(old_margin_box.total_right_padding)
end

Not only does it fix the example scenario I provided, the change also passes all tests in Asciidoctor PDF (which uses bounding boxes extensively).

[mojavelinux]

The same problem occurs in go_to_page. In fact, it can happen anytime generate_margin_box is called, given the right conditions.