backport fix for #2541 don't look for NULL glyph in fallback font as …

…this can impact line height

CHANGELOG.adoc

@@ -15,6 +15,7 @@ Bug Fixes::
 
 * support horizontal alignment on AsciiDoc table cell that only contains paragraphs (#2358)
 * don't allow AsciiDoc table cell to overrun bottom of page on which it fits (#2538)
+* don't look for NULL glyph in fallback font as this can impact line height (#2541)
 
 == 2.3.18 (2024-07-27) - @mojavelinux
 

docs/modules/theme/pages/prepare-custom-font.adoc

@@ -112,6 +112,9 @@ The fonts used by the default theme in Asciidoctor PDF provide all of these glyp
 If the .notdef glyph is non-empty (i.e., contains splines), it will be used as the default glyph when the document requests a character that's missing from the font.
 Unlike other glyphs, the .notdef glyph is referenced by name only, meaning it does not have a designated Unicode code point.
 
+Although Asciidoctor PDF uses the \u0000 character as a placeholder for inline anchors, the font is not required to provide this character.
+Asciidoctor PDF assumes that the primary font provides this character and uses the font metrics from that font as though it were there.
+
 If you're preparing a font for use in verbatim blocks (e.g., a listing block), you'll also need this range of characters:
 
 * \u2460 to \u2468 - circled numbers

lib/asciidoctor/pdf/ext/prawn/formatted_text/box.rb

@@ -61,8 +61,8 @@ def analyze_glyphs_for_fallback_font_support fragment_hash
     form_fragments_from_like_font_glyph_pairs font_glyph_pairs, fragment_hash
   end
 
-  # TODO: remove once Prawn 2.5 is released
   def find_font_for_this_glyph char, current_font, current_font_opts = {}, fallback_fonts_to_check = [], original_font = current_font
+    return current_font if char == ?\u0000 # never look for NUL character in fallback fonts as it's not rendered
     (doc = @document).font current_font, current_font_opts
     if doc.font.glyph_present? char
       current_font

spec/font_spec.rb

@@ -88,6 +88,28 @@
       (expect text[:font_name]).to eql 'NotoSerif-Bold'
     end
 
+    it 'should not look for NUL glyph in fallback font when missing from primary font' do
+      pdf_theme = {
+        extends: 'default',
+        font_catalog: {
+          'Noto Serif' => {
+            'normal' => 'notoserif-regular-subset.ttf',
+          },
+          'M+ 1p Fallback' => {
+            'normal' => 'mplus1p-regular-fallback.ttf',
+          },
+        },
+        base_font_family: 'M+ 1p Fallback',
+        font_fallbacks: ['Noto Serif'],
+      }
+      input = '. [[L1]]List item with anchor'
+      pdf = to_pdf input, analyze: true, pdf_theme: pdf_theme
+      marker, text = pdf.text
+      (expect marker[:font_name]).to eql 'mplus-1p-regular'
+      (expect text[:font_name]).to eql 'mplus-1p-regular'
+      (expect marker[:y]).to eql text[:y]
+    end
+
     it 'should include box drawing glyphs in bundled monospace font', visual: true do
       input_file = Pathname.new fixture_file 'box-drawing.adoc'
       to_file = to_pdf_file input_file, 'font-box-drawing.pdf'