herzenschein

Also, you should only use Layout attached properties when the object you're using it on is a child of a Layout, for example:

ApplicationWindow {
    ColumnLayout {
        anchors.fill: parent // Not a child of a layout, so you use anchors
        Controls.Button {
            Layout.fillWidth: true // It's a child of a layout, so you use Layout.fillWidth
        }
    }
}

So what you want to do is put a Kirigami.SearchField inside the global toolbar created by the Kirigami.Page. It's not a header.

In the first case you're putting it inside the page, and because it's a direct child of a Kirigami.Page, it is automatically laid out for you, which is fine, but not what you want.

In the second case you're putting it in the header of the ApplicationWindow, but you actually want to put in the global toolbar of the page, so it's not what you want.

In the third case you're just putting a SearchField before the initialPage, so it's loaded on the same level as the page, it's not what you want.

You can put it in the header property of the Kirigami.Page, but the header area of the page doesn't include the global toolbar, so they just happen to be close together by chance.

The way I'd have expected to do that would be to override the globalToolBarItem of the Kirigami.Page, but it's read only, so I don't really know how to help you with that. You should ask in the Kirigami Matrix room or in the Discuss forum.

I can't tell you why this is happening, but what is clearly happening is that your apps are being run under a nested kwin_wayland instance. It's as though you were running kwin_wayland krunner.

If anyone’s wondering, my main issue with Wayland was that it wasn’t setting the DISPLAY and WAYLAND_DISPLAY environment variables for some reason, and this would cause all kinds of software like Steam and Firefox to not even launch. I tried setting them manually but that didn’t go do well either.

My guess is that whatever fix you attempted here caused this, so you'd need to be more specific about what you tried.