Why is the x position of the QML TabButton negative?

This is my result when I compile it with Qt 5.15.2 :

and This is my result when I compile it with Qt 6.2.3 :

In Qt 6.2.3 the Results are correct but I lost the theme that was in Qt 5.15.2.look at the TabBar in both pictures.

So this is a bug in the version of Qt 5.15.2 and they fixed it in Qt6.

Now how can fix it in Qt 5.15.2, I add width: implicitWidth in each TabButton and test it again:

import QtQuick 2.15
import QtQuick.Window 2.15
import QtQuick.Controls 2.14
import QtQuick.Layouts 1.14

Window {
    height: 768
    width: 432
    visible: true
    title: qsTr("Hello World")

    TabBar {
        id: bar
        width: parent.width
        LayoutMirroring.enabled: false
        TabButton {
            id:homeButton
            width:  implicitWidth*2.3
            text: qsTr("Home")
            onClicked: console.log("Home: " + x)
        }
        TabButton {
            id:discoverButton
            width:  implicitWidth*2.3
            text: qsTr("Discover")
            onClicked: console.log("Discover: " + x)
        }
        TabButton {
            id:activityButton
            width:  implicitWidth*2.3
            text: qsTr("Activity")
            onClicked: console.log("Activity: " + x)
        }
    }


    StackLayout {
        width: parent.width
        currentIndex: bar.currentIndex
        Item {
            id: homeTab
        }
        Item {
            id: discoverTab
        }
        Item {
            id: activityTab
        }
    }

}

and that bug was fixed I guess that happens because you said width: parent.width and it decides x value after setting layout.

Contrary to the accepted answer it is not a bug. Qt documents says

originX : real

originY : real

These properties hold the origin of the content. This value always refers to the top-left position of the content regardless of layout direction.

This is usually (0,0), however ListView and GridView may have an arbitrary origin due to delegate size variation, or item insertion/removal outside the visible region.

So you actually need to subtract originX from the x to get the correct result. to access originX you could use ListView.view attached property here :

    TabButton {
        text: qsTr("Home")
        onClicked: console.log("Home: " + (x - ListView.view.originX))
    }
    TabButton {
        text: qsTr("Discover")
        onClicked: console.log("Discover: " + (x - ListView.view.originX))
    }
    TabButton { 
        text: qsTr("Activity")
        onClicked: console.log("Activity: " + (x - ListView.view.originX))
    }