Describe the bug
When a change is detected, the line is in bold and when I click on the "diff" button, I see the difference with the last time the watched page was checked.
Sometimes, several checks are done before I click on the diff button, and several changes happened on those several checks. In this case I expect to see the complete difference between the last time I clicked on "diff" and now. Instead, I only the diff with the last check.
The issue is that I can miss that something changed because it is not shown in the diff. How am I supposed to know which "compare from" date to select?
I could see that because I have set a notification system and I received several notifications for the several checks. And I didn't see all the changed content in the diff when I clicked on it

Version
Exact version in the top right area: 0.46.03

To Reproduce

1. Set a watcher on a page
2. Modify the page several times so the watcher sees several changes over several checks (do no click on the "diff" button yet)
3. Once 2. is done, click on the diff button
4. See that the diff page is showing only the diff from the last check

Expected behavior
I expect the "compare from" date to be automatically set to last date I saw the diff