PopupWindow focus handling

[tronical]

Following up on #1143 and related slightly to #2899 :

Suppose a PopupWindow declares a bunch of buttons:

    confirm_popup := PopupWindow {
        x: 40px;
        y: 100px;
        width: min(confirm_popup_layout.preferred-width, root.width - 80px);
        forward-focus: yes-button;
        Rectangle {
            background: root.background;
            border-color: confirm_popup_text.color;
            border-width: 1px;
        }
        confirm_popup_layout := Dialog {
            height:100%; width: 100%;
            confirm_popup_text := Text {
                text: "Some items are not done, are you sure you wish to quit?";
                wrap: word-wrap;
            }
            yes-button := StandardButton { kind: yes; clicked => { root.popup_confirmed(); } }
            StandardButton { kind: no; }
        }
    }

When this popup is down what should happen is:

1) The underlying window that created the popup does not show activate focus anymore 2) Focus is transferred to the popup and consequently to the yes/ok button. 3) Tab-focus should allow transferring focus between the buttons.

With Qt this almost works, except that the initial focus isn't shown.

With winit, which currently embeds the popup inline, none of the three points work.

[almindor]

One thing to note here is that in wayland/sway where focus can change by just moving the mouse, this bug has even bigger implications. Just moving the mouse over to another app/windows while a popup is out will close it without any way of getting a notification to the effect. This means that there's no good way "around" the issue so that user interaction loops can be "closed".

[ogoffart]

@almindor does it also happen when close-on-click is set to false?

[almindor]

@almindor does it also happen when close-on-click is set to false?

Yes