Closes #1982

changedetectionio/__init__.py

@@ -970,7 +970,7 @@ def changedetection_app(config=None, datastore_o=None):
         try:
             from_version_file_contents = watch.get_history_snapshot(dates[from_version_index])
         except Exception as e:
-            from_version_file_contents = "Unable to read to-version at index{}.\n".format(dates[from_version_index])
+            from_version_file_contents = f"Unable to read to-version at index {dates[from_version_index]}.\n"
 
         to_version = request.args.get('to_version')
         to_version_index = -1

changedetectionio/model/Watch.py

@@ -262,6 +262,25 @@ class model(dict):
         bump = self.history
         return self.__newest_history_key
 
+    # Given an arbitrary timestamp, find the closest next key
+    # For example, last_viewed = 1000 so it should return the next 1001 timestamp
+    #
+    # used for the [diff] button so it can preset a smarter from_version
+    @property
+    def get_next_snapshot_key_to_last_viewed(self):
+
+        """Unfortunately for now timestamp is stored as string key"""
+        keys = list(self.history.keys())
+        last_viewed = self.get('last_viewed')
+        prev_k=keys[0]
+        for k in sorted(keys, key=lambda x: int(x)):
+            if int(k) >= int(last_viewed):
+                return prev_k
+            prev_k = k
+
+        #@check this, return last if not found?
+        return keys[-1]
+
     def get_history_snapshot(self, timestamp):
         import brotli
         filepath = self.history[timestamp]

changedetectionio/templates/watch-overview.html

@@ -82,12 +82,15 @@
             </tr>
             {% endif %}
             {% for watch in (watches|sort(attribute=sort_attribute, reverse=sort_order == 'asc'))|pagination_slice(skip=pagination.skip) %}
+
+                {% set is_unviewed =  watch.newest_history_key| int > watch.last_viewed and watch.history_n>=2 %}
+
             <tr id="{{ watch.uuid }}"
                 class="{{ loop.cycle('pure-table-odd', 'pure-table-even') }} processor-{{ watch['processor'] }}
                 {% if watch.last_error is defined and watch.last_error != False %}error{% endif %}
                 {% if watch.last_notification_error is defined and watch.last_notification_error != False %}error{% endif %}
                 {% if watch.paused is defined and watch.paused != False %}paused{% endif %}
-                {% if watch.newest_history_key| int > watch.last_viewed and watch.history_n>=2 %}unviewed{% endif %}
+                {% if is_unviewed %}unviewed{% endif %}
                 {% if watch.uuid in queued_uuids %}queued{% endif %}">
                 <td class="inline checkbox-uuid" ><input name="uuids"  type="checkbox" value="{{ watch.uuid}} " > <span>{{ loop.index+pagination.skip }}</span></td>
                 <td class="inline watch-controls">
@@ -167,7 +170,13 @@
                        class="recheck pure-button pure-button-primary">{% if watch.uuid in queued_uuids %}Queued{% else %}Recheck{% endif %}</a>
                     <a href="{{ url_for('edit_page', uuid=watch.uuid)}}" class="pure-button pure-button-primary">Edit</a>
                     {% if watch.history_n >= 2 %}
+
+                        {%  if is_unviewed %}
+                           <a href="{{ url_for('diff_history_page', uuid=watch.uuid, from_version=watch.get_next_snapshot_key_to_last_viewed) }}" target="{{watch.uuid}}" class="pure-button pure-button-primary diff-link">Diff</a>
+                        {% else %}
                            <a href="{{ url_for('diff_history_page', uuid=watch.uuid)}}" target="{{watch.uuid}}" class="pure-button pure-button-primary diff-link">Diff</a>
+                        {% endif %}
+
                     {% else %}
                         {% if watch.history_n == 1 or (watch.history_n ==0 and watch.error_text_ctime )%}
                             <a href="{{ url_for('preview_page', uuid=watch.uuid)}}" target="{{watch.uuid}}" class="pure-button pure-button-primary">Preview</a>