diff --git a/Lib/idlelib/CodeContext.py b/Lib/idlelib/CodeContext.py
index e985969..5d67066 100644
--- a/Lib/idlelib/CodeContext.py
+++ b/Lib/idlelib/CodeContext.py
@@ -1,13 +1,12 @@
-"""CodeContext - Display the block context of code at top of edit window
+"""CodeContext - Extension to display the block context above the edit window
 
-Once code has scrolled off the top of the screen, it can be difficult
-to determine which block you are in.  This extension implements a pane
-at the top of each IDLE edit window which provides block structure
-hints.  These hints are the lines which contain the block opening
-keywords, e.g. 'if', for the enclosing block.  The number of hint lines
-is determined by the numlines variable in the CodeContext section of
-config-extensions.def. Lines which do not open blocks are not shown in
-the context hints pane.
+Once code has scrolled off the top of a window, it can be difficult to
+determine which block you are in.  This extension implements a pane at the top
+of each IDLE edit window which provides block structure hints.  These hints are
+the lines which contain the block opening keywords, e.g. 'if', for the
+enclosing block.  The number of hint lines is determined by the numlines
+variable in the CodeContext section of config-extensions.def. Lines which do
+not open blocks are not shown in the context hints pane.
 
 """
 import Tkinter
@@ -21,13 +20,14 @@
 UPDATEINTERVAL = 100 # millisec
 FONTUPDATEINTERVAL = 1000 # millisec
 
-getspacesfirstword = lambda s, c=re.compile(r"^(\s*)(\w*)"): c.match(s).groups()
+getspacesfirstword =\
+                   lambda s, c=re.compile(r"^(\s*)(\w*)"): c.match(s).groups()
 
 class CodeContext:
     menudefs = [('options', [('!Code Conte_xt', '<<toggle-code-context>>')])]
 
-    numlines = idleConf.GetOption("extensions", "CodeContext",
-                                  "numlines", type="int", default=3)
+    context_depth = idleConf.GetOption("extensions", "CodeContext",
+                                       "numlines", type="int", default=3)
     bgcolor = idleConf.GetOption("extensions", "CodeContext",
                                  "bgcolor", type="str", default="LightGray")
     fgcolor = idleConf.GetOption("extensions", "CodeContext",
@@ -37,13 +37,13 @@
         self.text = editwin.text
         self.textfont = self.text["font"]
         self.label = None
-        # self.info holds information about the context lines of line number
-        # self.lastfirstline. The information is a tuple of the line's
-        # indentation, the line's text and the keyword at the beginning of the
-        # line, as returned by get_line_info. At the beginning of the list
-        # there's a dummy line, which starts the "block" of the whole document.
+        # self.info is a list of (line number, indent level, line text, block
+        # keyword) tuples providing the block structure associated with
+        # self.topvisible (the linenumber of the line displayed at the top of
+        # the edit window). self.info[0] is initialized as a 'dummy' line which
+        # starts the toplevel 'block' of the module.
         self.info = [(0, -1, "", False)]
-        self.lastfirstline = 1
+        self.topvisible = 1
         visible = idleConf.GetOption("extensions", "CodeContext",
                                      "visible", type="bool", default=False)
         if visible:
@@ -56,7 +56,7 @@
     def toggle_code_context_event(self, event=None):
         if not self.label:
             self.label = Tkinter.Label(self.editwin.top,
-                                      text="\n" * (self.numlines - 1),
+                                      text="\n" * (self.context_depth - 1),
                                       anchor="w", justify="left",
                                       font=self.textfont,
                                       bg=self.bgcolor, fg=self.fgcolor,
@@ -77,6 +77,7 @@
 
         If the line does not start a block, the keyword value is False.
         The indentation of empty lines (or comment lines) is INFINITY.
+
         """
         text = self.text.get("%d.0" % linenum, "%d.end" % linenum)
         spaces, firstword = getspacesfirstword(text)
@@ -87,64 +88,69 @@
             indent = len(spaces)
         return indent, text, opener
 
-    def interesting_lines(self, firstline, stopline=1, stopindent=0):
+    def get_context(self, new_topvisible, stopline=1, stopindent=0):
+        """Get context lines, starting at new_topvisible and working backwards.
+
+        Stop when stopline or stopindent is reached. Return a tuple of context
+        data and the indent level at the top of the region inspected.
+
         """
-        Find the context lines, starting at firstline.
-        Will not return lines whose index is smaller than stopline or whose
-        indentation is smaller than stopindent.
-        stopline should always be >= 1, so the dummy block start will never
-        be returned (This function doesn't know what to do about it.)
-        Returns a list with the context lines, starting from the first (top),
-        and a number which all context lines above the inspected region should
-        have a smaller indentation than it.
-        """
+        assert stopline > 0
         lines = []
         # The indentation level we are currently in:
         lastindent = INFINITY
         # For a line to be interesting, it must begin with a block opening
         # keyword, and have less indentation than lastindent.
-        for line_index in xrange(firstline, stopline-1, -1):
-            indent, text, opener = self.get_line_info(line_index)
+        for linenum in xrange(new_topvisible, stopline-1, -1):
+            indent, text, opener = self.get_line_info(linenum)
             if indent < lastindent:
                 lastindent = indent
                 if opener in ("else", "elif"):
                     # We also show the if statement
                     lastindent += 1
-                if opener and line_index < firstline and indent >= stopindent:
-                    lines.append((line_index, indent, text, opener))
+                if opener and linenum < new_topvisible and indent >= stopindent:
+                    lines.append((linenum, indent, text, opener))
                 if lastindent <= stopindent:
                     break
         lines.reverse()
         return lines, lastindent
 
-    def update_label(self):
-        """Update the CodeContext label, if needed.
+    def update_code_context(self):
+        """Update context information and lines visible in the context pane.
+
         """
-        firstline = int(self.text.index("@0,0").split('.')[0])
-        if self.lastfirstline == firstline:
+        new_topvisible = int(self.text.index("@0,0").split('.')[0])
+        if self.topvisible == new_topvisible:      # haven't scrolled
             return
-        if self.lastfirstline < firstline:
-            lines, lastindent = self.interesting_lines(firstline,
-                                                       self.lastfirstline)
+        if self.topvisible < new_topvisible:       # scroll down
+            lines, lastindent = self.get_context(new_topvisible,
+                                                 self.topvisible)
+            # retain only context info applicable to the region
+            # between topvisible and new_topvisible:
             while self.info[-1][1] >= lastindent:
                 del self.info[-1]
-            self.info.extend(lines)
-        else:
+        elif self.topvisible > new_topvisible:     # scroll up
             stopindent = self.info[-1][1] + 1
-            while self.info[-1][0] >= firstline:
+            # retain only context info associated
+            # with lines above new_topvisible:
+            while self.info[-1][0] >= new_topvisible:
                 stopindent = self.info[-1][1]
                 del self.info[-1]
-            lines, lastindent = self.interesting_lines(
-                firstline, self.info[-1][0]+1, stopindent)
-            self.info.extend(lines)
-        self.lastfirstline = firstline
-        lines = [""] * max(0, self.numlines - len(self.info)) + \
-                [x[2] for x in self.info[-self.numlines:]]
-        self.label["text"] = '\n'.join(lines)
+            lines, lastindent = self.get_context(new_topvisible,
+                                                 self.info[-1][0]+1,
+                                                 stopindent)
+        self.info.extend(lines)
+        self.topvisible = new_topvisible
+
+        # empty lines in context pane:
+        context_strings = [""] * max(0, self.context_depth - len(self.info))
+        # followed by the context hint lines:
+        context_strings += [x[2] for x in self.info[-self.context_depth:]]
+        self.label["text"] = '\n'.join(context_strings)
 
     def timer_event(self):
         if self.label:
-            self.update_label()
+            self.update_code_context()
         self.text.after(UPDATEINTERVAL, self.timer_event)
 
     def font_timer_event(self):