KEPUB Output: Add an option to generate KEPUB files that have better text justification at the cost of gaps in highlighting when used on the Kobo. Fixes #2107778 [Native KEPUB conversion causing some justification issues](https://bugs.launchpad.net/calibre/+bug/2107778)

Note that this option also affects on-the-fly generation of KEPUB done
by the Kobo driver.

src/calibre/devices/kobo/driver.py

@@ -2347,7 +2347,10 @@ class KOBOTOUCH(KOBO):
         return result
 
     def _kepubify(self, path, name, mi) -> None:
+        from calibre.ebooks.conversion.config import load_defaults
         from calibre.ebooks.oeb.polish.kepubify import kepubify_path, make_options
+        prefs = load_defaults('kepub_output')
+        prefer_justification = prefs.get('kepub_prefer_justification', False)
         debug_print(f'Starting conversion of {mi.title} ({name}) to kepub')
         opts = make_options(
             extra_css=self.extra_css or '',
@@ -2359,6 +2362,7 @@ class KOBOTOUCH(KOBO):
             hyphenation_limit_lines=self.get_pref('hyphenation_limit_lines'),
             remove_at_page_rules=self.extra_css_options.get('has_atpage', False),
             remove_widows_and_orphans=self.extra_css_options.get('has_widows_orphans', False),
+            prefer_justification=prefer_justification,
         )
         try:
             kepubify_path(path, outpath=path, opts=opts, allow_overwrite=True)

src/calibre/ebooks/conversion/config.py

@@ -282,7 +282,7 @@ OPTIONS = {
             'preserve_cover_aspect_ratio', 'epub_flatten', 'epub_version', 'epub_max_image_size',),
 
         'kepub': (
-            'dont_split_on_page_breaks', 'flow_size', 'kepub_max_image_size',
+            'dont_split_on_page_breaks', 'flow_size', 'kepub_max_image_size', 'kepub_prefer_justification',
             'kepub_affect_hyphenation', 'kepub_disable_hyphenation', 'kepub_hyphenation_min_chars',
             'kepub_hyphenation_min_chars_before', 'kepub_hyphenation_min_chars_after', 'kepub_hyphenation_limit_lines',
         ),

src/calibre/ebooks/conversion/plugins/epub_output.py

@@ -611,6 +611,16 @@ class KEPUBOutput(OutputFormatPlugin):
             help=max_image_size_help
         ),
 
+        OptionRecommendation(name='kepub_prefer_justification', recommended_value=False,
+            help=_(
+                'The KEPUB renderer on the Kobo has a bug when text justification is turned on.'
+                ' It will either not justify text properly or when highlighting there will be gaps'
+                ' between neighboring highlighted parts of text. By default, calibre generates'
+                ' KEPUB that avoid the highlighting gaps at the expense of worse text justification.'
+                ' This option reverses that tradeoff. Use this option if you use justification when'
+                ' reading on your Kobo device.'
+        )),
+
         OptionRecommendation(name='kepub_affect_hyphenation', recommended_value=False,
             help=_('Modify how hyphenation is performed for this book. Note that hyphenation'
                    ' does not perform well for all languages, as it depends on the dictionaries'
@@ -653,6 +663,7 @@ class KEPUBOutput(OutputFormatPlugin):
                 hyphenation_min_chars_before=opts.kepub_hyphenation_min_chars_before,
                 hyphenation_min_chars_after=opts.kepub_hyphenation_min_chars_after,
                 hyphenation_limit_lines=opts.kepub_hyphenation_limit_lines,
+                prefer_justification=opts.kepub_prefer_justification,
             )
             kepubify_container(container, kopts)
             container.commit()

src/calibre/ebooks/oeb/polish/kepubify.py

@@ -609,6 +609,7 @@ def make_options(
     hyphenation_min_chars_before: int = 3,
     hyphenation_min_chars_after: int = 3,
     hyphenation_limit_lines: int = 2,
+    prefer_justification: bool = False,
 
     remove_widows_and_orphans: bool | None = None,
     remove_at_page_rules: bool | None = None,
@@ -652,7 +653,7 @@ h1, h2, h3, h4, h5, h6, td {{
 '''
     return Options(
         extra_css=extra_css, hyphenation_css=hyphen_css, remove_widows_and_orphans=remove_widows_and_orphans,
-        remove_at_page_rules=remove_at_page_rules)
+        remove_at_page_rules=remove_at_page_rules, prefer_justification=prefer_justification)
 
 
 def profile():

src/calibre/gui2/convert/kepub_output.ui

@@ -63,21 +63,21 @@
    <item row="2" column="1">
     <widget class="QLineEdit" name="opt_kepub_max_image_size"/>
    </item>
-   <item row="3" column="0" colspan="2">
+   <item row="4" column="0" colspan="2">
     <widget class="QCheckBox" name="opt_kepub_affect_hyphenation">
      <property name="text">
       <string>Enable/disable &amp;hyphenation for this book</string>
      </property>
     </widget>
    </item>
-   <item row="4" column="0">
+   <item row="5" column="0">
     <widget class="QCheckBox" name="opt_kepub_disable_hyphenation">
      <property name="text">
       <string>&amp;Prevent all hyphenation</string>
      </property>
     </widget>
    </item>
-   <item row="5" column="0">
+   <item row="6" column="0">
     <widget class="QLabel" name="label_2">
      <property name="text">
       <string>Hyphenation: &amp;minimum word length:</string>
@@ -87,7 +87,7 @@
      </property>
     </widget>
    </item>
-   <item row="5" column="1">
+   <item row="6" column="1">
     <widget class="QSpinBox" name="opt_kepub_hyphenation_min_chars">
      <property name="specialValueText">
       <string>Disabled</string>
@@ -97,7 +97,7 @@
      </property>
     </widget>
    </item>
-   <item row="6" column="0">
+   <item row="7" column="0">
     <widget class="QLabel" name="label_3">
      <property name="text">
       <string>Hyphenation: minimum characters &amp;before:</string>
@@ -107,14 +107,14 @@
      </property>
     </widget>
    </item>
-   <item row="6" column="1">
+   <item row="7" column="1">
     <widget class="QSpinBox" name="opt_kepub_hyphenation_min_chars_before">
      <property name="suffix">
       <string> characters</string>
      </property>
     </widget>
    </item>
-   <item row="7" column="0">
+   <item row="8" column="0">
     <widget class="QLabel" name="label_5">
      <property name="text">
       <string>Hyphenation: minimum characters &amp;after:</string>
@@ -124,14 +124,14 @@
      </property>
     </widget>
    </item>
-   <item row="7" column="1">
+   <item row="8" column="1">
     <widget class="QSpinBox" name="opt_kepub_hyphenation_min_chars_after">
      <property name="suffix">
       <string> characters</string>
      </property>
     </widget>
    </item>
-   <item row="8" column="0">
+   <item row="9" column="0">
     <widget class="QLabel" name="label_6">
      <property name="text">
       <string>Hyphenation: &amp;limit lines:</string>
@@ -141,14 +141,14 @@
      </property>
     </widget>
    </item>
-   <item row="8" column="1">
+   <item row="9" column="1">
     <widget class="QSpinBox" name="opt_kepub_hyphenation_limit_lines">
      <property name="suffix">
       <string> lines</string>
      </property>
     </widget>
    </item>
-   <item row="9" column="0">
+   <item row="10" column="0">
     <spacer name="verticalSpacer">
      <property name="orientation">
       <enum>Qt::Vertical</enum>
@@ -161,6 +161,13 @@
      </property>
     </spacer>
    </item>
+   <item row="3" column="0" colspan="2">
+    <widget class="QCheckBox" name="opt_kepub_prefer_justification">
+     <property name="text">
+      <string>Prefer improved text &amp;justification to proper highlighting</string>
+     </property>
+    </widget>
+   </item>
   </layout>
  </widget>
  <resources/>

src/pyj/book_list/conversion_widgets.pyj

@@ -558,6 +558,7 @@ def kepub_output(container):
     g.appendChild(checkbox('dont_split_on_page_breaks', _('Do not &split on page breaks')))
     g.appendChild(int_spin('flow_size', _('Split files &larger than:'), unit='KB', max=1000000, step=20))
     g.appendChild(lineedit('kepub_max_image_size', _('Shrink &images larger than:')))
+    g.appendChild(lineedit('kepub_prefer_justification', _('Prefer improved text justification to proper highlighting')))
     g.appendChild(checkbox('kepub_affect_hyphenation', _('Enable/disable &hyphenation for this book')))
     g.appendChild(checkbox('kepub_disable_hyphenation', _('&Prevent all hyphenation')))
     c = _('characters')