resolves asciidoctor#2547 allow value of base-font-size-min and <cate…

…gory>-font-size-min theme keys to be relative

CHANGELOG.adoc

@@ -38,6 +38,7 @@ Improvements::
 * drop support for the unmaintained payment font (`pf`) for use in font-based icons
 * refactor formatted text transform to simplify how inner space is collapsed; verify only inner hard breaks are preserved
 * allow relative font size for sub and sup to be set independently; support combined setting for backwards compatibility
+* allow value of `base-font-size-min` and `<category>-font-size-min` theme keys to be relative (e.g., 0.75em) (#2547)
 
 Bug Fixes::
 

docs/modules/theme/pages/base.adoc

@@ -59,11 +59,11 @@ base:
   font-size: 10.5
 
 |font-size-min
-|xref:language.adoc#values[Number] +
+|xref:text.adoc#font-size[Font size] +
 (default: `6`)
 |[source]
 base:
-  font-size-min: $base-font-size * 0.75
+  font-size-min: 0.75rem
 
 |font-style
 |xref:text.adoc#font-style[Font style] +

docs/modules/theme/pages/text.adoc

@@ -43,6 +43,9 @@ The font size may be specified either as a fixed value (e.g., `12`) or a relativ
 Font sizes are most often specified as a fixed value, though there are important cases when a relative value is more suitable.
 The font size always resolves to a point value when text is written to the PDF.
 
+IMPORTANT: The `base-font-size` must always be a number.
+That's because the value of this key is the reference value on which relative font values (e.g., `0.75rem`) are based.
+
 If the font size is specified as a number, or a number with a fixed unit of measurement (e.g., `px`), that value is converted to points.
 The conversion to points is done when the theme is loaded.
 The `font-size` key on the `base` category, as well as min, large, and small font size keys, must always be specified as a fixed value.

lib/asciidoctor/pdf/converter.rb

@@ -4836,14 +4836,27 @@ def compute_autofit_font_size fragments, category
         padding = expand_padding_value @theme[%(#{category}_padding)]
         if actual_width > (available_width = bounds.width - padding[3].to_f - padding[1].to_f)
           adjusted_font_size = ((available_width * font_size).to_f / actual_width).truncate 4
-          if (min = @theme[%(#{category}_font_size_min)] || @theme.base_font_size_min) && adjusted_font_size < min
+          if (min = @theme[%(#{category}_font_size_min)] || @theme.base_font_size_min) && adjusted_font_size < (min = resolve_font_size min)
             min
           else
             adjusted_font_size
           end
         end
       end
 
+      def resolve_font_size value
+        return value unless ::String === value
+        if value.end_with? 'rem'
+          @root_font_size * value.to_f
+        elsif value.end_with? 'em'
+          font_size * value.to_f
+        elsif value.end_with? '%'
+          font_size * (value.to_f / 100)
+        else
+          value.to_f
+        end
+      end
+
       def consolidate_ranges nums
         if nums.size > 1
           prev = nil

spec/listing_spec.rb

@@ -341,6 +341,30 @@
     end
   end
 
+  it 'should allow base minimum font size to be specified relative to base font size' do
+    pdf = to_pdf <<~'END', pdf_theme: { base_font_size: 12, base_font_size_min: '0.5rem' }, analyze: true
+    [%autofit]
+    ----
+    play_symbol = (node.document.attr? 'icons', 'font') ? %(<font name="fas">#{(icon_font_data 'fas').unicode 'play'}</font>) : RightPointer
+    ----
+    END
+
+    (expect pdf.text).to have_size 1
+    (expect pdf.text[0][:font_size].floor).to be 7
+  end
+
+  it 'should allow base minimum font size to be specified relative to current font size' do
+    pdf = to_pdf <<~'END', pdf_theme: { base_font_size: 15, code_font_size: 12, base_font_size_min: '0.5em' }, analyze: true
+    [%autofit]
+    ----
+    play_symbol = (node.document.attr? 'icons', 'font') ? %(<font name="fas">#{(icon_font_data 'fas').unicode 'play'}</font>) : RightPointer
+    ----
+    END
+
+    (expect pdf.text).to have_size 1
+    (expect pdf.text[0][:font_size].floor).to be 7
+  end
+
   it 'should use base font color if font color is not specified' do
     pdf = to_pdf <<~'END', pdf_theme: { base_font_color: 'AA0000', code_font_color: nil }, analyze: true
     before