perf: improve scoped slots change detection accuracy

[yyx990803]

Ensure that state mutations that only affect parent scope only trigger parent update and does not affect child components with only scoped slots.

Implications:

• For optimal performance, scoped slots should always be preferred. (In v3 all slots are unified to work like scoped slots so that will be the default)

• In 2.6, any slots using the new v-slot syntax are compiled as scopedSlots internally. This doesn't affect the child if it's using template, since <slot/> can render both scoped and normal slots, but if the child is using render functions, it is recommended to always use this.$scopedSlots. It is safe to do so in 2.6 since all normal slots are proxied on this.$scopedSlots as well.

• Unfortunately, any child components with static slot content still need to be forced updated. This means the common use case of <parent><child></child></parent> doesn't benefit from this change, unless the default slot is explicitly forced into a scoped slot by using <parent v-slot:default><child></child></parent>. (We cannot directly force all slots into scoped slots as that would break existing render function code that expects the slot to be present on this.$slots instead of `this.$scopedSlots)

[codecov-io]

Codecov Report

Merging #9371 into dev will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev    #9371      +/-   ##
==========================================
+ Coverage   97.33%   97.34%   +<.01%     
==========================================
  Files         107      107              
  Lines        4544     4551       +7     
==========================================
+ Hits         4423     4430       +7     
  Misses        121      121

Impacted Files	Coverage Δ
src/core/vdom/helpers/normalize-scoped-slots.js	94.44% <100%> (+0.32%)	⬆️
src/compiler/codegen/index.js	100% <100%> (ø)	⬆️
src/compiler/parser/index.js	98.88% <100%> (+0.01%)	⬆️
src/core/instance/render-helpers/resolve-slots.js	100% <100%> (ø)	⬆️
src/core/instance/lifecycle.js	100% <100%> (ø)	⬆️

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[johnsoncheg]

Did the last sentence of the implications said that it's not recommended to use the new syntax to force transfer the normal slot to scoped slot according to the phase We cannot directly force all slots into scoped slots as that would break existing render function code that expects the slot to be present on this.$slotsinstead ofthis.$scopedSlots` @yyx990803

[Yuliang-Lee]

Why static slot need to be forceUpdate?I can't think of any scenes that need forceUpdate

[marsprince]

Why static slot need to be forceUpdate?I can't think of any scenes that need forceUpdate

I think the reason that child components need to be forceUpdate is vue doesn't "know" if static slot changed. Slot vnode is rendered in parent but used in child. We can't compare two vnodes to see if it changed or unchanged.