Note: the Widget Container control (xe: widgetContainer) in the XPages extension library is roughly equivalent to the OneUI control. You can see that in the Extension Library Demo App: XPagesExt.nsf / OneUI_WidgetContainer.xsp It might be better to customize the page using this control rather than the xp: control.
In addition, the OneUI 3 document that you are linking to, to the right of the page, 25% down the page, has a Subject drop-down list where you select "default" or "gen2". The behavior you are asking for is similar to the behavior of "gen2" (similar to OneUI 2), while your screenshot of how it looks is closer to the "default" behavior (OneUI 3). You can learn how to enable the appearance of gen2.
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