Don't instanciate ListView items that are just above the viewport when scrolling down

[DataTriny]

Fixes #7381

If an item can just fit above the viewport while scrolling, it is now not instanciated anymore as it is fully hidden. The issue occurs frequently when using the arrow keys to jump through the items, since the scroll delta will usually be the height of exactly one item. This previously meant that an extra node would be added at the start of ListViews accessibility tree.

[ogoffart]

Thanks for the patch.
This might workaround the issue #7381 for the case of ListView, so i'm going to include it.
But I think this won't work for the general case with the Flickable.

For example, consider this:

import { AboutSlint, Button, VerticalBox } from "std-widgets.slint";

export component Demo {
    height: 300px;
    width: 300px;
    Flickable {
        VerticalBox {
            AboutSlint { }
            for i in 50 : Text {
                text: "Hello " + i;
            }
        }
    }
}

Then all the Text are going to be instantiated regardless if they are visible or not.
I believe the right fix would be somehow to exclude the clipped item from the accessible tree when we visit the tree. @tronical: opinions?

I think it might make sense to not generate the item if it is out even if it is by one pixel.
Although it might have implication for the computation of the average height of items if item have different height, and the more item we have the better. But we can still merge this patch.

[tronical]

Thanks for the patch. This might workaround the issue #7381 for the case of ListView, so i'm going to include it. But I think this won't work for the general case with the Flickable.

For example, consider this:

import { AboutSlint, Button, VerticalBox } from "std-widgets.slint";

export component Demo {
    height: 300px;
    width: 300px;
    Flickable {
        VerticalBox {
            AboutSlint { }
            for i in 50 : Text {
                text: "Hello " + i;
            }
        }
    }
}

Then all the Text are going to be instantiated regardless if they are visible or not. I believe the right fix would be somehow to exclude the clipped item from the accessible tree when we visit the tree. @tronical: opinions?

I agree, I think we need to perform a visibility check. We do this for the testing backend as well, perhaps accessible_descendants() needs to be changed and perform a visibility check like here ?

[DataTriny]

My initial change was indeed very specific. I've implemented a general fix but we're now back to the point where listviews are excluded from the focus chain, even if no scrolling is performed. My understanding is that it is due to this hack used throughout the codebase:

FocusScope {
    x: 0;
    width: 0;  // Do not react on clicks
}

Shouldn't there be another property on FocusScope to ignore mouse clicks?