Allow diffing a change before saving

Why

When editing a page, a crucial feature is to be able to view the diffs of the change before saving the change.

For example:

1. I've updated a page with some small corrections. (ex: north -> south and seperate -> separate).
2. I've also accidentally deleted a few sentences in a different section of the page without realizing it.
3. Being able to diff the changes would allow me to catch the unintentional changes before saving them

How

Confluence allows viewing diffs of pages after they are saved (under Tools->Page History) The same diff functionality just needs to be re-used for providing a pre-save diff.

Additional Notes

• Older versions of confluence (with wiki markup), MediaWiki/wikipedia, and most other top notch wiki solutions provide this functionality
• MediaWiki calls this functionality "Show Changes"
• This would also allow the possibility of avoid the multiple editor race condition:
  • Editor1 makes a change, but has not saved yet
  • Editor2 begins editing
  • Editor1 saves
  • Editor2 saves and inadvertently rolls back Editor1's change
  • If Editor2 had been able to preview the diff before saving, they could have seen Editor1's changes in there and avoided wiping them out.