This seems to be a problem of UltraEdit for Windows of the currently latest releases which are 26.00.0.62 or 26.00.0.72. I have never seen this wrong behavior before.

• Select some lines whereby first and last line are not selected 'from beginning' or 'to end' of line.
• Press several times the button for Add line comment.
• The first line of selection is suddenly not selected anymore.
• Then the second line.
• Then the third and so on.

See screencast from UE 26.00.0.72. This happens with different wordfiles, i.e. with different languages.

It is definitely a bug. Please report it by email to IDM support.

I can reproduce this issue with UE v26.00.0.62 and v26.00.0.72, but everything is fine and works as expected with UE v26.00.0.48 and former versions. The beginning and end of selected block should not change on inserting the line comment character(s) which means the start and end columns of the selection should be automatically increased by UltraEdit.

I reported it, it was confirmed and it is fixed by IDM in UE 26.10.0.14.