Edit book: A new option to show a configurable number lines above the current line when syncing the position of the preview panel to the current position in the code editor (under Preview settings in the Editor preferences). Fixes #1908929 [Enhancement request for seeing code changes in file previewer.](https://bugs.launchpad.net/calibre/+bug/1908929)

src/calibre/gui2/tweak_book/__init__.py

@@ -38,6 +38,7 @@ d['preview_standard_font_family'] = 'serif'
 d['preview_base_font_size'] = 18
 d['preview_mono_font_size'] = 14
 d['preview_minimum_font_size'] = 8
+d['preview_sync_context'] = 0
 d['preview_background'] = 'auto'
 d['preview_foreground'] = 'auto'
 d['preview_link_color'] = 'auto'

src/calibre/gui2/tweak_book/preferences.py

@@ -381,6 +381,11 @@ class PreviewSettings(BasicSettings):  # {{{
         w = self('preview_minimum_font_size')
         w.setMinimum(4), w.setMaximum(100), w.setSuffix(' px')
         l.addRow(_('Mi&nimum font size:'), w)
+        w = self('preview_sync_context')
+        w.setMinimum(0), w.setMaximum(10), w.setSuffix(' ' + _('lines'))
+        w.setToolTip('<p>' + _(
+            'Number of lines that are shown above the current line when syncing the text shown in the preview panel to the cursor position in the code view'))
+        l.addRow(_('Visible lines above s&ync point:'), w)
         l.addRow(_('Background color:'), self.color_override('preview_background'))
         l.addRow(_('Foreground color:'), self.color_override('preview_foreground'))
         l.addRow(_('Link color:'), self.color_override('preview_link_color'))

src/calibre/gui2/tweak_book/preview.py

@@ -341,7 +341,7 @@ class WebPage(QWebEnginePage):
             if lnum is None:
                 return
             tags = [x.lower() for x in tags]
-            self.bridge.go_to_sourceline_address.emit(lnum, tags)
+            self.bridge.go_to_sourceline_address.emit(lnum, tags, tprefs['preview_sync_context'])
 
     def split_mode(self, enabled):
         if self.bridge.ready:

src/pyj/editor.pyj

@@ -40,34 +40,53 @@ def find_containing_block(elem):
         elem = elem.parentNode
     return elem
 
-def scroll_to_node(node):
+
+def line_height():
+    ans = line_height.ans
+    if not ans:
+        ds = window.getComputedStyle(document.body)
+        try:
+            # will fail if line-height = "normal"
+            lh = float(ds.lineHeight)
+        except:
+            try:
+                lh = 1.2 * float(ds.fontSize)
+            except:
+                lh = 15
+        ans = line_height.ans = max(5, lh)
+    return ans
+
+
+def scroll_to_node(node, sync_context):
     if node is document.body:
         window.scrollTo(0, 0)
     else:
         node.scrollIntoView()
+        if sync_context:
+            window.scrollBy(0, -sync_context * line_height())
 
 
 state = {'blocks_found': False, 'in_split_mode': False}
 
 
 
-def go_to_line(lnum):
+def go_to_line(lnum, sync_context):
     for node in document.querySelectorAll(f'[data-lnum="{lnum}"]'):
         if is_hidden(node):
             continue
-        scroll_to_node(node)
+        scroll_to_node(node, sync_context)
         break
 
 @from_python
-def go_to_sourceline_address(sourceline, tags):
+def go_to_sourceline_address(sourceline, tags, sync_context):
     nodes = document.querySelectorAll(f'[data-lnum="{sourceline}"]')
     for index in range(nodes.length):
         node = nodes[index]
         if index >= tags.length or node.tagName.toLowerCase() is not tags[index]:
             break
         if index == tags.length - 1 and not is_hidden(node):
-            return scroll_to_node(node)
-    go_to_line(sourceline)
+            return scroll_to_node(node, sync_context)
+    go_to_line(sourceline, sync_context)
 
 def line_numbers():
     found_body = False