widgets.diff: Save and restore selection across updates

git-cola's inotify integration causes it to automatically refresh itself
when files are changed by another process.

This can be problematic because refreshing the "diff" viewer causes it
to lose the user's selection.

Teach the "diff" viewer to save and restore the selection when its
contents change.  When no selection is present, restore the previous
cursor position.

Closes #155

Reported-by: Peter Júnoš <petoju@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>

cola/widgets/diff.py

@@ -7,11 +7,11 @@
 from cola import cmds
 from cola import qtutils
 from cola.cmds import run
-from cola.widgets import defs
 from cola.widgets.text import DiffTextEdit
 
 
 class DiffEditor(DiffTextEdit):
+
     def __init__(self, parent):
         DiffTextEdit.__init__(self, parent)
         self.model = model = cola.model()
@@ -143,13 +143,40 @@ def setPlainText(self, text):
         highlight = (self.mode != self.model.mode_none and
                      self.mode != self.model.mode_untracked)
         self.highlighter.set_enabled(highlight)
+
         scrollbar = self.verticalScrollBar()
         if scrollbar:
             scrollvalue = scrollbar.value()
-        if text is not None:
-            DiffTextEdit.setPlainText(self, text)
-            if scrollbar:
-                scrollbar.setValue(scrollvalue)
+        else:
+            scrollvalue = None
+
+        if text is None:
+            return
+
+        offset, selection = self.offset_and_selection()
+        old_text = unicode(self.toPlainText())
+
+        DiffTextEdit.setPlainText(self, text)
+
+        # If the old selection exists in the new text then
+        # re-select it.
+        if selection and selection in text:
+            idx = text.index(selection)
+            cursor = self.textCursor()
+            cursor.setPosition(idx)
+            cursor.setPosition(idx + len(selection),
+                               QtGui.QTextCursor.KeepAnchor)
+            self.setTextCursor(cursor)
+
+        # Otherwise, if the text is identical and there
+        # is no selection then restore the cursor position.
+        elif text == old_text:
+            cursor = self.textCursor()
+            cursor.setPosition(offset)
+            self.setTextCursor(cursor)
+
+        if scrollbar and scrollvalue is not None:
+            scrollbar.setValue(scrollvalue)
 
     def offset_and_selection(self):
         cursor = self.textCursor()

share/doc/git-cola/relnotes.rst

@@ -9,6 +9,14 @@ Usability, bells and whistles
 
 Fixes
 -----
+* The inotify auto-refresh feature makes it difficult to select text in
+  the "diff" editor when files are being continually modified by another
+  process.  The auto-refresh causes it to lose the currently selected text,
+  which is not wanted.  We now avoid this problem by saving and restoring
+  the selection when refreshing the editor.
+
+  http://github.com/git-cola/git-cola/issues/155
+
 * Fixed the Alt+D Diffstat shortcut.
 
   http://github.com/git-cola/git-cola/issues/159

share/doc/git-cola/thanks.rst

@@ -33,6 +33,7 @@ Thanks
 * Michael Homer
 * Omega Weapon
 * Paolo G. Giarrusso
+* Peter Júnoš
 * Rustam Safin
 * Stefan Naewe
 * Steffen Prohaska