`ContentPresenter` should fall back to `Content` when value is not available

[MartinZikmund]

Current behavior

AppBarButton has the following in the template:

<ContentPresenter x:Name="Content" Content="{TemplateBinding Icon}" Foreground="{TemplateBinding Foreground}" />

When Icon is not set, it magically falls back to parent's Content instead. Compare:

See ContentPresenter.cpp:

pdpTarget = GetPropertyByIndexInline(KnownPropertyIndex::ContentPresenter_Content);
            IFCEXPECT(pdpTarget);
            if (IsPropertyDefault(pdpTarget) && !IsPropertyTemplateBound(pdpTarget))
            {
                const CDependencyProperty* pdpSource = pTemplatedParent->GetPropertyByIndexInline(KnownPropertyIndex::ContentControl_Content);
                IFCEXPECT(pdpSource);

                IFC(SetTemplateBinding(pdpTarget, pdpSource));
                needsRefresh = true;
            }

Expected behavior

No response

How to reproduce it (as minimally and precisely as possible)

No response

Workaround

No response

Works on UWP/WinUI

None

Environment

No response

NuGet package version(s)

No response

Affected platforms

No response

IDE

No response

IDE version

No response

Relevant plugins

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Anything else we need to know?

No response

[Youssef1313]

All that should be fixed by #18455, if you want to take over the PR :)