Preview panel: Improve robustness of resource loading from the book

Ensure that resources are always loaded from the current container, even
if the current container was changed since the base URL was specified.
While this is unlikely to happen, it is best to be robust.

src/calibre/ebooks/oeb/polish/container.py

@@ -310,8 +310,8 @@ class Container(object):  # {{{
             for elem in self.parsed(name).xpath('//*[@src]'):
                 yield (elem.get('src'), elem.sourceline, 0) if get_line_numbers else elem.get('src')
 
-    def abspath_to_name(self, fullpath):
+    def abspath_to_name(self, fullpath, root=None):
-        return self.relpath(os.path.abspath(fullpath)).replace(os.sep, '/')
+        return self.relpath(os.path.abspath(fullpath), base=root).replace(os.sep, '/')
 
     def name_to_abspath(self, name):
         return os.path.abspath(join(self.root, *name.split('/')))

src/calibre/gui2/tweak_book/preview.py

@@ -238,6 +238,7 @@ class NetworkAccessManager(QNetworkAccessManager):
 
     def __init__(self, *args):
         QNetworkAccessManager.__init__(self, *args)
+        self.current_root = None
         self.cache = QNetworkDiskCache(self)
         self.setCache(self.cache)
         self.cache.setCacheDirectory(PersistentTemporaryDirectory(prefix='disk_cache_'))
@@ -251,7 +252,7 @@ class NetworkAccessManager(QNetworkAccessManager):
                 path = path[1:]
             c = current_container()
             try:
-                name = c.abspath_to_name(path)
+                name = c.abspath_to_name(path, root=self.current_root)
             except ValueError:  # Happens on windows with absolute paths on different drives
                 name = None
             if c.has_name(name):
@@ -306,6 +307,14 @@ class WebPage(QWebPage):
         self.mainFrame().javaScriptWindowObjectCleared.connect(self.init_javascript)
         self.init_javascript()
 
+    @dynamic_property
+    def current_root(self):
+        def fget(self):
+            return self.networkAccessManager().current_root
+        def fset(self, val):
+            self.networkAccessManager().current_root = val
+        return property(fget=fget, fset=fset)
+
     def javaScriptConsoleMessage(self, msg, lineno, source_id):
         prints('preview js:%s:%s:'%(unicode(source_id), lineno), unicode(msg))
 
@@ -408,6 +417,11 @@ class WebView(QWebView):
             page margins and embedded fonts that use font name aliasing.
 
             '''))
+        self.page().current_root = None
+
+    def setUrl(self, qurl):
+        self.page().current_root = current_container().root
+        return QWebView.setUrl(self, qurl)
 
     def inspect(self):
         self.inspector.parent().show()
@@ -565,6 +579,10 @@ class Preview(QWidget):
         self.view.clear()
         self.current_name = None
 
+    @property
+    def current_root(self):
+        return self.view.page().current_root
+
     @property
     def is_visible(self):
         return actions['preview-dock'].isChecked()