Show diff in entry preview when conflicts are resolved

[koppor]

1. Entry is modified externally
2. JabRef shows "modified" notification
   
3. "Review changes" brings up "External Changes Resolver"
   

Following issues:

• The entry preview shows the new entry only - the old one is not shown
• The entry preview does not show a diff

I would like to see the additions marked in green, the deletions marked in red

Similar as we have in the Merge Dialog

It would be nice if this was possible in the preview. If not, maybe the "Merge Entries" dialog can be displayed -- showing the rows with differences only!

[HoussemNasri]

It's already possible to see diffs in the three-way-merge dialog. Just click "merge..."

[Siedlerchr]

• Show diff in preview
• Show only those fields that differ in the merge dialog

For the moment I would suggest we first implement the second one. The first one seems a bit more complicated.

[HoussemNasri]

Maybe we should add an option to "show only different fields" in this header to the three-way-merge dialog. And enable the option by default when the dialog is opened from the external change resolver.

[koppor]

I think, I had following case in mind: JabRef notifies me that the library was changed externally. I think: WTF and want to get back to work as fast as possible. Thus, I open the dialog and want quickly to see the changes. Thus, I don't want to click an additional button opening another dialog 😇

The second point, the user-button-configurable "Show only those fields that differ in the merge dialog" is good! 👍

[Siedlerchr]

In principle the showing only the changed rows works. Only problem as I set them with "managed" the style or column constraints are not reapplied...

Refs https://github.com/JabRef/jabref/pull/9533/files#diff-fe83eaf6bb2edc0cfce71bdaf73c8997794d0016ac3540da82e7bfe5884a340eR167-R190

[HoussemNasri]

Got it working by removing and adding nodes from the GridPane, but the code is so ugly...

[Siedlerchr]

Remaning issue:
Show diff in preview

[koppor]

Another example:

When taking a closer look, date was added with trash content. Hard to see.

[koppor]

I still have to learn that I have to click "Merge..." 😅