Should isolation be added conditionally?

[khushalsagar]

Currently we only add the isolation: isolate property if the ::page-transition-container has both images. Since it's only needed to blend both and unnecessarily adding it is perf overhead. Is that ok or should we always add it.

For resolving issue

[jakearchibald]

It seems like a fairly straight forward UA optimisation, unless I'm missing something:

1. If the element has isolation: isolate:
   1. If the element contains 0 renderable children: No need to isolate.
   2. Else, if the element contain no renderable descendants with a non-normal mix-blend-mode (ignoring descendant isolated elements), then create a stacking context, but there's no need to apply isolation.
   3. Else, if the element contains 1 child, and none of its descendants have non-normal mix-blend-mode (ignoring descendant isolated elements, then create a stacking context, but there's no need to apply isolation.

[khushalsagar]

Yeah I'm going to see if we can optimize this internally. We need to detect whether isolation and mix-blend-mode are a no-op. The paint stack assumes that mix-blend-mode on any element implies that an offscreen render pass is needed, likely coz its rare that a node with a non-default mix-blend-mode is already a texture/single image. But we should be able to detect this case and optimize.