acdream/tests/AcDream.App.Tests/UI/UiTemplateListBoxFlushPreservingScrollTests.cs

using AcDream.App.UI;
using AcDream.App.UI.Layout;

namespace AcDream.App.Tests.UI;

/// <summary>
/// FA3 carry-forward 1 (folded into Campaign FA slice FA4): unlike
/// <see cref="UiTemplateListBox.Flush"/>, <see cref="UiTemplateListBox.FlushPreservingScroll"/>
/// must not reset the scroll offset to 0 — it exists specifically so a
/// roster rebuild triggered by a server-side change (not user action)
/// doesn't yank a scrolled-down user back to the top.
/// </summary>
public sealed class UiTemplateListBoxFlushPreservingScrollTests
{
    private static UiTemplateListBox MakeListBox(float width, float height)
    {
        var info = new ElementInfo { Id = 0x10000279u, Type = 5u };
        return new UiTemplateListBox(
            info,
            _ => (0u, 0, 0),
            new[] { new UiTemplateListEntry(0x21000030u, 0x10000281u) },
            scrollbarElementId: 0x1000027Au)
        {
            Width = width,
            Height = height,
        };
    }

    private static void AddRows(UiTemplateListBox box, int count, float rowHeight = 40f)
    {
        for (int i = 0; i < count; i++)
            box.AddPrebuiltRow(new UiText { Width = box.Width, Height = rowHeight });
    }

    [Fact]
    public void Flush_ResetsScrollToZero_TheOrdinaryContract()
    {
        var box = MakeListBox(200f, 100f);
        AddRows(box, 10); // 400px of content in a 100px viewport
        box.ViewportForTest!.ApplyAnchor(box.Width, box.Height);
        box.ViewportForTest!.LayoutScrollableChildren();
        box.Scroll.SetScrollY(150);
        Assert.Equal(150, box.Scroll.ScrollY);

        box.Flush();

        Assert.Equal(0, box.Scroll.ScrollY);
    }

    [Fact]
    public void FlushPreservingScroll_KeepsTheScrollOffset()
    {
        var box = MakeListBox(200f, 100f);
        AddRows(box, 10); // 400px of content in a 100px viewport
        box.ViewportForTest!.ApplyAnchor(box.Width, box.Height);
        box.ViewportForTest!.LayoutScrollableChildren();
        box.Scroll.SetScrollY(150);
        Assert.Equal(150, box.Scroll.ScrollY);

        box.FlushPreservingScroll();

        Assert.Equal(150, box.Scroll.ScrollY);

        // Rebuild the SAME row count — the offset should still be valid
        // (and stay put) once the next layout pass recomputes content
        // height against the rebuilt rows.
        AddRows(box, 10);
        box.ViewportForTest!.LayoutScrollableChildren();
        Assert.Equal(150, box.Scroll.ScrollY);
    }

    [Fact]
    public void FlushPreservingScroll_ClampsToTheNewShorterContent()
    {
        var box = MakeListBox(200f, 100f);
        AddRows(box, 10); // 400px of content
        box.ViewportForTest!.ApplyAnchor(box.Width, box.Height);
        box.ViewportForTest!.LayoutScrollableChildren();
        box.Scroll.SetScrollY(300); // near the bottom of the 10-row content

        box.FlushPreservingScroll();
        // Rebuild with far FEWER rows (e.g. most of the fellowship left).
        AddRows(box, 2); // 80px of content, less than the 100px viewport
        box.ViewportForTest!.LayoutScrollableChildren();

        // Clamped to the new MaxScroll (0, since content < viewport) —
        // never left dangling past the real content, and never throws.
        Assert.Equal(0, box.Scroll.ScrollY);
    }

    [Fact]
    public void FlushPreservingScroll_DormantBox_IsANoOp()
    {
        var box = MakeListBox(200f, 100f);
        // No rows ever added — the viewport was never created.
        box.FlushPreservingScroll();
        Assert.Null(box.ViewportForTest);
    }
}