fix: improve streaming text insertion and stop button functionality

Streaming Text Insertion:
- Disable widget updates during text insertion to prevent auto-scroll
- Append using separate QTextCursor on document (doesn't affect visible cursor)
- Only auto-scroll to bottom if user was already at bottom (watching stream)
- Preserve user's scroll position when they scroll up to read earlier content
- Qt version compatibility for QTextCursor.End enum (Qt5/Qt6)

Stop Button Fix:
- Add cancellation check inside streaming generator consumption loop
- Check cancel_request() on each character/chunk during streaming
- Raise TranslationCanceled immediately when stop is requested
- Apply to both single translation and batch mode streaming
- Fixes infinite 'Stopping...' state with large merged translations

Scroll Synchronization:
- Keep simple pixel-based scroll sync (most stable)
- Note: Minor drift can occur with word wrap when languages have different lengths
- Alternative sync methods (line-based, percentage-based) had worse issues

Author: seidnerj

advanced.py

@@ -6,7 +6,7 @@
     QPlainTextEdit, QPushButton, QSplitter, QLabel, QThread, QLineEdit,
     QGridLayout, QProgressBar, pyqtSignal, pyqtSlot, QPixmap, QEvent,
     QStackedWidget, QSpacerItem, QTabWidget, QCheckBox,
-    QComboBox, QSizePolicy)
+    QComboBox, QSizePolicy, QTextCursor)
 from calibre.constants import __version__  # type: ignore
 from calibre.gui2 import I  # type: ignore
 from calibre.utils.localization import _  # type: ignore
@@ -973,6 +973,7 @@ def layout_review(self):
         self.review_splitter.setSizes(_size)
 
         def synchronizeScrollbars(editors):
+            """Sync scrollbars between editors using simple pixel-based approach."""
             for editor in editors:
                 for other_editor in editors:
                     if editor != other_editor:
@@ -1139,7 +1140,31 @@ def streaming_translation(data):
             elif isinstance(data, Paragraph):
                 self.table.setCurrentItem(self.table.item(data.row, 0))
             else:
-                translation_text.insertPlainText(data)
+                # Check if user is at bottom (watching stream)
+                scrollbar = translation_text.verticalScrollBar()
+                was_at_bottom = scrollbar.value() >= scrollbar.maximum() - 10
+
+                # Disable updates to prevent auto-scroll/flicker
+                translation_text.setUpdatesEnabled(False)
+                saved_position = scrollbar.value()
+
+                # Append to document using cursor at end
+                doc = translation_text.document()
+                cursor = QTextCursor(doc)
+                end_position = getattr(QTextCursor.MoveOperation, 'End', None) or QTextCursor.End
+                cursor.movePosition(end_position)
+                cursor.insertText(data)
+
+                # Restore scroll position before re-enabling updates
+                if not was_at_bottom:
+                    scrollbar.setValue(saved_position)
+
+                # Re-enable updates - this triggers repaint
+                translation_text.setUpdatesEnabled(True)
+
+                # Scroll to bottom only if user was watching
+                if was_at_bottom:
+                    scrollbar.setValue(scrollbar.maximum())
         self.trans_worker.streaming.connect(streaming_translation)
 
         def modify_translation():

lib/translation.py

@@ -169,14 +169,23 @@ def translate_paragraph(self, paragraph):
                 temp = ''
                 clear = True
                 for char in translation:
+                    # Check for cancellation during streaming
+                    if self.cancel_request():
+                        raise TranslationCanceled(_('Translation canceled.'))
                     if clear:
                         self.streaming('')
                         clear = False
                     self.streaming(char)
                     time.sleep(0.05)
                     temp += char
             else:
-                temp = ''.join([char for char in translation])
+                # For batch mode, still check cancellation periodically
+                temp_chars = []
+                for char in translation:
+                    if self.cancel_request():
+                        raise TranslationCanceled(_('Translation canceled.'))
+                    temp_chars.append(char)
+                temp = ''.join(temp_chars)
             translation = temp
         translation = self.glossary.restore(translation)
         paragraph.translation = translation.strip()