Merge Conflicts Resolver does not allow editing in "Result after merge"

### Apache NetBeans version

Apache NetBeans 20

### What happened

This used to work. it was fixed in this archived bug: https://bz.apache.org/netbeans/show_bug.cgi?id=141198
but seems to have reappeared

### Language / Project Type / NetBeans Component

_No response_

### How to reproduce

1. Merge a branch generating a git conflict.
2. Select a change to accept or both
3. Try to change the result in the lower pane.
4. An edit cursor appears but editing is not allowed

### Did this work correctly in an earlier version?

Apache NetBeans 13 or earlier

### Operating System

Mac OS 14.1.2 (23B2091)

### JDK

OpenJDK Runtime Environment AdoptOpenJDK (build 15.0.2+7)

### Apache NetBeans packaging

Apache NetBeans provided installer

### Anything else

_No response_

### Are you willing to submit a pull request?

Yes

No file or test is named. First reproduce the conflict in the Merge Conflicts Resolver and inspect the lower “Result after merge” pane after accepting changes; done means the result remains editable and the regression scenario works as described.

Written by the indexing model from the issue text.