Incorrect toolbar location when restored via `ToolStripManager.LoadSettings`

[RussKie]

Presumably all versions:

• .NET Framework: 4.6.1 (tested)
• .NET Version: 5.0 (tested)

Problem description:

Originally reported in gitextensions/gitextensions#8680.

If an application has multiple toolbars, and those are placed at an arbitrary location and persisted via ToolStripManager.SaveSettings, not all toolbars will be correctly located once ToolStripManager.LoadSettings is called.
E.g. on save:

On load:

Expected behavior:

The location of the toolbars persisted in the config must be preserved.

Minimal repro:

1. clone https://github.com/RussKie/Test-ToolStripLoad
2. Run the app, align both toolbars to the left one under another.
3. Stop the app.
4. Open the user.config file, located somethere at C:\Users\<user>\AppData\Local\ToolstripLoad\ToolstripLoad_Url_<random string>\1.0.0.0, check that both toolbars' locations are like this:

        <System.Windows.Forms.ToolStripSettings.toolsettings.ToolStripFilters>
            <setting name="Name" serializeAs="String">
                <value>ToolStripFilters</value>
            </setting>
            <setting name="Location" serializeAs="String">
                <value>7, 25</value>
            </setting>
        </System.Windows.Forms.ToolStripSettings.toolsettings.ToolStripFilters>
        <System.Windows.Forms.ToolStripSettings.toolsettings.ToolStripMain>
            <setting name="Name" serializeAs="String">
                <value>ToolStripMain</value>
            </setting>
            <setting name="Location" serializeAs="String">
                <value>7, 0</value>
            </setting>
        </System.Windows.Forms.ToolStripSettings.toolsettings.ToolStripMain>

4. Run the app again and observe the bottom toolbar shifted by few pixels to the right.
5. Close the app and see that the location value for ToolStripFilters has changed.

[RussKie]

Spent some time debugging this, and it looks like this line causes the issue:

this.toolPanel.TopToolStripPanel.Padding = new System.Windows.Forms.Padding(4, 0, 4, 0);

If the panel's padding is 0, then there is no shift.

Stepping through the code I ascertained the information from the config is loaded and applied correctly to each individual toolstrip. However then we call
https://github.com/dotnet/winforms/blob/74844307652966164160773a1f0287be7248e591/src/System.Windows.Forms/src/System/Windows/Forms/ToolStripSettingsManager.cs#L175
...the TopToolStripPanel's padding is
a) not applied to the first toolstrip, and
b) (incorrectly) applied to the following panels.

So far I wasn't able to figure the exact logic, but I suspect the root cause is buried somewhere in here:
https://github.com/dotnet/winforms/blob/74844307652966164160773a1f0287be7248e591/src/System.Windows.Forms/src/System/Windows/Forms/Layout/FlowLayout.cs#L98

[RussKie]

It is possible to achieve the desired UI layout with the following dirty workaround:

        public override void OnLoad(EventArgs e)
        {
            // Restore the toolbars layout
            ToolStripManager.LoadSettings(this, "toolsettings");

            // Apply the padding
            this.toolPanel.TopToolStripPanel.Padding = new System.Windows.Forms.Padding(4, 0, 4, 0);
        }

        protected override void OnFormClosing(FormClosingEventArgs e)
        {
            // Reset padding to zero before saving
            this.toolPanel.TopToolStripPanel.Padding = new System.Windows.Forms.Padding(0, 0, 0, 0);

            // Persist the toolbars layout
            ToolStripManager.SaveSettings(this, "toolsettings");
        }