tweak history prefix search (up/^p) in qtconsole

moving the cursor around the line could result in
weird inconsistencies in the prefix.

This simplifies the logic by always using the cursor position
to set the history prefix,
and better determine when the history prefix has actually changed.

IPython/frontend/qt/console/history_console_widget.py

@@ -76,9 +76,21 b' class HistoryConsoleWidget(ConsoleWidget):'
             # Set a search prefix based on the cursor position.
             col = self._get_input_buffer_cursor_column()
             input_buffer = self.input_buffer
-            if self._history_index == len(self._history) or \
-                    (self._history_prefix and col != len(self._history_prefix)):
+            # use the *shortest* of the cursor column and the history prefix
+            # to determine if the prefix has changed
+            n = min(col, len(self._history_prefix))
+            
+            # prefix changed, restart search from the beginning
+            if (self._history_prefix[:n] != input_buffer[:n]):
                 self._history_index = len(self._history)
+            
+            # the only time we shouldn't set the history prefix
+            # to the line up to the cursor is if we are already
+            # in a simple scroll (no prefix),
+            # and the cursor is at the end of the first line
+            first_line = input_buffer.split('\n', 1)[0]
+            if self._history_index == len(self._history) or \
+                not (self._history_prefix == '' and col == len(first_line)):
                 self._history_prefix = input_buffer[:col]
 
             # Perform the search.