install/60334: sysinst(8): partition list is incorrectly updated on UEFI mode when the swap partition is deleted

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>Number:         60334
>Category:       install
>Synopsis:       sysinst(8): partition list is incorrectly updated on UEFI mode when the swap partition is deleted
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    install-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Jun 15 08:35:00 +0000 2026
>Originator:     Andrius V
>Release:        NetBSD 11.0 RC4
>Organization:
>Environment:
>Description:
I noticed an issue in sysinst on amd64 systems booting in UEFI mode after selecting "Use default partition sizes".
When the swap partition (the third partition) is deleted, the remaining two partitions become identical to the first partition.

On BIOS systems, the default partition layout contains only two partitions, and the issue could not be reproduced after creating and deleting a third partition.

By default it lists three partitions:
a: MSDOS (for uefi boot)
b: FFSv2ea / (root)
c: swap

Deleting the swap partition refreshes the list to two same MSDOS partitions:
a: MSDOS
b: MSDOS
(same offsets and sizes)

Deleting MSDOS or root partition works OK, the list stays correct.
>How-To-Repeat:
Boot on amd64 USB flash drive flash drive in (UEFI boot)

Select options:
Installation messages in English
unchanged
Install NetBSD to hard disk
Yes
sd0 (install disk in my case)
This is the correct geometry
Use default partition sizes
Delete swap partition
Observe two same MSDOS partitions (a/b).
>Fix: