Can't actually fix the differences (user error?)

[SanderBouwhuis]

I'm probably missing something stupid, but I can't seem to fix the changes between files.

Describe the bug
I am trying to fix a file with the difference of another file. But, it doesn't work. It seems the files are read-only or something?!?

To Reproduce
Steps to reproduce the behavior:

1. Choose two files in the Solution explorer
2. Right mouse click, choose Compare/Selected Files
3. Go to a difference
4. Try to fix the file.

Expected behavior
Being able to 'copy/synchronize' the lines of the compared files.

Screenshots

Additional context
The file on the right side is editable, but the file on the left seems 'read-only'?!?

Edit:
I now see what may be the bug:
The file on the left side is not the file I chose, but it seems to be a 'copy'?

Name of the original file I selected : CGrid_ByDate.cpp
Name of the file in compare window : CGrid_ByDate - Copy.cpp