Custom style properties can be incorrect when composing elements of the same type

[sorvell]

Example:

<x-stuff>
  <x-stuff></x-stuff>
</x-stuff>

Due to the way style property data is shimmed, elements in the inner x-stuffin the example above may be styled with property data calculated for the outer x-stuff element.

We can potentially address this by having a stronger (slower) scoping mechanism for shimming property data which is employed in cases where same type nesting is detected.

This issue came up while debugging: #2218

[TimvdLippe]

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