"Create a multipolygon" creates multiple relations

[GerdP]

What steps will reproduce the problem?

1. select two closed ways with building=yes, not overlapping
2. click on "Multi" button in the side pane (tooltip says "Create a multipolygon from selected objects")

What is the expected result?

One type=multipolygon relation with two role=outer members, one command on the undo/redo stack

What happens instead?

Two multipolygon relations, each containing one ring, two commands on the undo/redo stack

Please provide any additional information below. Attach a screenshot if possible.

If this is intended then I don't understand the idea behind it.

Relative:URL: ^/trunk
Repository:UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Last:Changed Date: 2024-02-05 12:56:34 +0100 (Mon, 05 Feb 2024)
Revision:18969
Build-Date:2024-02-06 02:30:58
URL:https://josm.openstreetmap.de/svn/trunk

Identification: JOSM/1.5 (18969 en) Windows 10 64-Bit
OS Build number: Windows 10 Pro 2009 (19045)
Memory Usage: 279 MB / 1888 MB (168 MB allocated, but free)
Java version: 17.0.8+7-LTS, Azul Systems, Inc., OpenJDK 64-Bit Server VM
Look and Feel: com.sun.java.swing.plaf.windows.WindowsLookAndFeel
Screen: \Display0 1920×1080 (scaling 1.00×1.00)
Maximum Screen Size: 1920×1080
Best cursor sizes: 16×16→32×32, 32×32→32×32
System property file.encoding: Cp1252
System property sun.jnu.encoding: Cp1252
Locale info: en_DE
Numbers with default locale: 1234567890 -> 1234567890
VM arguments: [-Djpackage.app-version=1.5.18789, --add-modules=java.scripting,java.sql,javafx.controls,javafx.media,javafx.swing,javafx.web, --add-exports=java.base/sun.security.action=ALL-UNNAMED, --add-exports=java.desktop/com.sun.imageio.plugins.jpeg=ALL-UNNAMED, --add-exports=java.desktop/com.sun.imageio.spi=ALL-UNNAMED, --add-opens=java.base/java.lang=ALL-UNNAMED, --add-opens=java.base/java.nio=ALL-UNNAMED, --add-opens=java.base/jdk.internal.loader=ALL-UNNAMED, --add-opens=java.base/jdk.internal.ref=ALL-UNNAMED, --add-opens=java.desktop/javax.imageio.spi=ALL-UNNAMED, --add-opens=java.desktop/javax.swing.text.html=ALL-UNNAMED, --add-opens=java.prefs/java.util.prefs=ALL-UNNAMED, -Djpackage.app-path=%UserProfile%\AppData\Local\JOSM\HWConsole.exe]
Dataset consistency test: No problems found

Plugins:
+ OpeningHoursEditor (36196)
+ RoadSigns (36196)
+ SimplifyArea (36209)
+ apache-commons (36176)
+ buildings_tools (36200)
+ comfort0 (36200)
+ o5m (36126)
+ pbf (36176)
+ poly (36126)
+ reltoolbox (36200)
+ reverter (36196)
+ splinex (36126)
+ undelete (36126)
+ utilsplugin2 (36200)

Validator rules:
+ d:\java_tools\JOSM\mygeometry.mapcss
+ https://josm.openstreetmap.de/josmfile?page=Rules/GermanySpecific&zip=1
+ c:\josm\core\resources\data\validator\geometry.mapcss

Last errors/warnings:
- 00000.491 W: extended font config - overriding 'filename.Myanmar_Text=mmrtext.ttf' with 'MMRTEXT.TTF'
- 00000.493 W: extended font config - overriding 'filename.Mongolian_Baiti=monbaiti.ttf' with 'MONBAITI.TTF'
- 00000.816 E: java.security.KeyStoreException: Windows-ROOT not found. Cause: java.security.NoSuchAlgorithmException: Windows-ROOT KeyStore not available

[GerdP]

I just noticed that this problem only occurs when preference reltoolbox.multipolygon.allowsplit is set to true.
No idea why I changed that from the default value false.

[GerdP]

When reltoolbox.multipolygon.allowsplit is set to false, only one command is added to the undo/redo stack but still two multipolygons are created. This only changes when I change preference reltoolbox.multipolygon.tags to false, but then the multipolygon has no tags.
Is there a documentation about the settings?
I just try to understand in what case the plugin will provide other results compared to the core method and why.

[GerdP]

While working on #23521 I learned that Create Multipolygon in this tool has a rather special meaning:
It is meant to convert one or more closed ways to multipolygon relation(s), one relation for each way. The way(s) may be split so that the multipolygons share the same segments. If a closed way has common segments with another multipolygon, that multipolygon may also be changed.
(Probably not only) in Germany this type of mapping is discouraged, many call it "multipolygonitis" and invest a lot of time to rebuild the areas with closed ways. I wonder if this tool should show a warning message in some areas when ways were split?

One more remark: If the selected ways require a role inner the plugin calls the core method to create a single multipolygon from all selected ways.