[meld] gutterrendererchunk: Add some unpleasant style context caching

[Kai Willadsen <kaiw src gnome org>]

commit 3286e9d66eb8ddaee706fab8d31cc96e152991b5
Author: Kai Willadsen <kai willadsen gmail com>
Date:   Sun Oct 2 09:31:59 2016 +1000

    gutterrendererchunk: Add some unpleasant style context caching
    
    As the docstring expounds up, this is unpleasant and theoretically very
    fragile, but in practice is almost certain to be totally fine. It is,
    in other words, the worst kind of hack.

 meld/gutterrendererchunk.py |   34 +++++++++++++++++++++++++++-------
 1 files changed, 27 insertions(+), 7 deletions(-)
---
diff --git a/meld/gutterrendererchunk.py b/meld/gutterrendererchunk.py
index a44e387..1dd0ca6 100644
--- a/meld/gutterrendererchunk.py
+++ b/meld/gutterrendererchunk.py
@@ -36,6 +36,27 @@ def load(icon_name):
     return icon_theme.load_icon(icon_name, LINE_HEIGHT, 0)
 
 
+def get_background_rgba(renderer):
+    '''Get and cache the expected background for the renderer widget
+
+    Current versions of GTK+ don't paint the background of text view
+    gutters with the actual expected widget background, which causes
+    them to look wrong when put next to any other widgets. This hack
+    just gets the background from the renderer's view, and then caches
+    in for performance, and on the basis that all renderers will be
+    assigned to similarly-styled views. This is fragile, but the
+    alternative is really significantly slower.
+    '''
+    global _background_rgba
+    if _background_rgba is None:
+        if renderer.props.view:
+            stylecontext = renderer.props.view.get_style_context()
+            background_set, _background_rgba = (
+                stylecontext.lookup_color('theme_bg_color'))
+    return _background_rgba
+_background_rgba = None
+
+
 class MeldGutterRenderer(object):
 
     def on_setting_changed(self, meldsettings, key):
@@ -77,19 +98,18 @@ class MeldGutterRenderer(object):
         chunk_index = self.linediffer.locate_chunk(self.from_pane, line)[0]
 
         if chunk_index is not None:
-            self._chunk = self.linediffer.get_chunk(
+            chunk = self.linediffer.get_chunk(
                 chunk_index, self.from_pane, self.to_pane)
 
-            if self.props.view.current_chunk_check(self._chunk):
+            if self.props.view.current_chunk_check(chunk):
                 background_rgba = self.fill_colors['current-chunk-highlight']
             else:
-                background_rgba = self.fill_colors[self._chunk[0]]
+                background_rgba = self.fill_colors[chunk[0]]
         else:
-            self._chunk = None
+            chunk = None
             # TODO: Remove when fixed in upstream GTK+
-            stylecontext = self.props.view.get_style_context()
-            background_set, background_rgba = (
-                stylecontext.lookup_color('theme_bg_color'))
+            background_rgba = get_background_rgba(self)
+        self._chunk = chunk
         self.set_background(background_rgba)