Fixes issue #153

[ccjmne]

Fixes issue #153 by creating a stacking context on the [main] content so that all its potentially z-indexed children are stacked within a context scoped to that element.

Introduced as little change as possible

The #scrim overlay sits over it simply by being added next -- current behavior inchanged.
Stacking context created with:

will-change: transform;

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[ccjmne]

I signed it!

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