Why is this a problem? Because it breaks manual bindings for the checked property: Button's code alters the checked state when seemingly no one asked it to. Consider the following code (adapted from plasma-pa applet):

SmallToolButton {
    id: muteButton

    icon.name: Icon.name(item.model.Volume, item.model.Muted, isPlayback)
    text: item.model.Muted ? "Unmute" : "Mute"

    onClicked: item.model.Muted = !item.model.Muted;
    checked: item.model.Muted
}

This component is supposed to manage its checked state externally, since audio sources/sinks may switch their muted state by other means (e.g. Mute key on a keyboard, or via other software). But what happens if a user double clicks it?

Starting with qtquickcontrols2@09476c590ab13623130d5ac79045fa68e504be96 the clicked() is no more emitted after doubleClicked(), but the nextCheckState() is still called regardless!

There are two problems at once:

1. There's no way to disable internal management of the checked property. Setting checkable to false does not help, because it would be immediately reset to true the next time setChecked is called.
2. Double click triggers checked state change.
3. Double click is emitted even when there are no listeners. But his is not a problem anymore, since the fix was merged for Qt 6 as qtdeclarative@778e08a4061a87b5b2d6c4cee082ded74e1bcc64