Fix the expander never collapsing its content

[FrayxRulez]

Problem

SettingsExpander.OnExpandedChanged guards the scoped batch's completion against a newer
toggle:

var tracker = _tracker++;
...
batch.Completed += (s, args) =>
{
    if (_tracker == tracker)
    {
        PopupRoot.Visibility = _expanded ? Visibility.Visible : Visibility.Collapsed;
    }
};

_tracker++ returns the value before the increment, so by the time the handler runs the
comparison is (n + 1) == n. It is never true, and the body never executes — on any toggle,
not just a superseded one.

That handler is the only thing that applies Visibility.Collapsed, and PopupRoot.Visibility
is set to Visible unconditionally a few lines above:

PopupHost.Height = newValue ? double.NaN : 0;
PopupRoot.Margin = new Thickness(0, 0, 0, newValue ? 0 : -PopupRoot.ActualHeight);
PopupRoot.Visibility = Visibility.Visible;

So a collapsed expander keeps its content Visible in the tree, still measured and arranged
on every pass, hidden only by Height = 0 on the host and a negative bottom margin.

Fix

Pre-increment, so tracker identifies this run:

var tracker = ++_tracker;

The guard then means what it was written to mean — apply the final visibility unless a newer
toggle has started since this batch began. The stale-completion case it was added for still
works, because a subsequent toggle bumps _tracker past the captured value.

History

The guard arrived in a3a98e6d31 together with the switch from the captured newValue to the
_expanded field, which is what makes the intent unambiguous. The later c67940f5e6 ("Fix
expander layout cycle") removed a SizeChanged handler that rewrote the same margin, and is
unrelated to this.

Verification

Not built or run — a UWP/.NET Native build is not available in the environment this was
prepared in. The edited file was checked with Roslyn (CSharpSyntaxTree.ParseText) and parses
with no syntax errors; that confirms syntax only, not type checking. Worth a visual check that
collapsing still animates rather than snapping, since the collapse now ends in a real
visibility change.

🤖 Generated with Claude Code