[gen] Bugfixes in historization.

gen/mixins/__init__.py

@@ -1041,7 +1041,8 @@ class BaseMixin:
             else:
                 for event in history:
                     if (event['action'] == '_datachange_') and \
-                       (fieldName in event['changes']): return True
+                       (fieldName in event['changes']) and \
+                       event['changes'][fieldName][0]: return True
 
     def getHistory(self, startNumber=0, reverse=True, includeInvisible=False,
                    batchSize=5):
@@ -1079,7 +1080,7 @@ class BaseMixin:
                        (field.format == gen.String.XHTML):
                         # For rich text fields, instead of simply showing the
                         # previous value, we propose a diff with the next
-                        # version.
+                        # version, excepted if the previous value is empty.
                         if field.isEmptyValue(oldValue[0]):
                             val = '-'
                         else:

shared/diff.py

@@ -660,6 +660,14 @@ class HtmlDiff:
 
     def get(self):
         '''Produces the result.'''
+        # Normally, if self.old is empty, the whole self.new should be
+        # considered as inserted text. But see this line:
         if not self.old or not self.old.strip(): return self.new
+        # Why? This is for avoiding problems in the case of cumulative diffs.
+        # A cumulative diff means: calling HtmlDiff with, as old value, the
+        # result of a previous call to HtmlDiff. In this case, if the whole text
+        # is already considered as inserted, we will already have overlaps in
+        # the next diff. Overlaps are hard to manage, so we avoid to get them
+        # as a starting point when computing cumulative diffs.
         return self.getHtmlDiff(self.old, self.new, '\n')
 # ------------------------------------------------------------------------------