Steps to reproduce

Save this as changes.vim.

func HandleChange(bufnr, start, end, added, changes)
    let g:report_number += 1
    let msg = printf(
        \ '%d: lines = %d -> %d, added = %d',
        \ g:report_number, a:start, a:end, a:added)
    call add(g:reports, msg)

    for d in a:changes
        let msg = printf(
            \ '    lines = %d -> %d, added = %d',
            \ d['lnum'], d['end'], d['added'])
        call add(g:reports, msg)
    endfor
endfunc

let g:report_number = 0
let g:reports = []

winpos 20 40
set lines=89
set columns=120
edit text1.txt
call listener_add('HandleChange', bufnr())

8,9 !cat text2.txt
undo
call listener_flush(bufnr())

new
call append(0, g:reports)

Create text1.txt with content:

Line 1
Line 2
Line 3
Line 4
Line 5
Line 6
Line 7
Line 8
Line 9
Line 10

and text2.txt:

Line a
Line b
Line c
Line d
Line e

Run [g]vim -c 'source changes.vim'.

Expected behaviour

Just before the undo command is executed the buffer contains 13 lines:

Line 1
Line 2
Line 3
Line 4
Line 5
Line 6
Line 7
Line a
Line b
Line c
Line d
Line e
Line 10

The 3rd report is:

3: lines = 8 -> 15, added = -3
      lines = 8 -> 8, added = 2
      lines = 10 -> 15, added = -5

I would have expected the report to start "3: lines = 8 -> 13, added = -3".

I may just be misunderstanding, but this seems to make the overall start, end and added values of little value and require looping over the list of individual change reports.

I confess that I am not sure if this is a bug, or just a feature that needs clarifying in the help.

Version of Vim

IMproved 9.1 patches: 1-1813

Environment

Linux, running in Gvim.

Logs and stack traces