⚡[RUMF-1344] Serialize scroll positions only for full snapshots

[bcaudan]

Motivation

Accessing scroll positions of DOM elements during serialization is costly so let's avoid to do it when we can.

Changes

First step, when serializing:

• DOM mutations => don't serialize scroll positions
• full snapshots => access scroll attributes to serialize scroll positions

Follow up in next PR:

• access scroll attributes only on initial full snapshot

Testing

• Local
• Staging
• Unit
• End to end

---

I have gone over the contributing documentation.

[codecov-commenter]

Codecov Report

Merging #1670 (df8bb4a) into main (8dec744) will increase coverage by 0.00%.
The diff coverage is 64.70%.

@@           Coverage Diff           @@
##             main    #1670   +/-   ##
=======================================
  Coverage   90.71%   90.72%           
=======================================
  Files         127      127           
  Lines        4696     4699    +3     
  Branches     1051     1052    +1     
=======================================
+ Hits         4260     4263    +3     
  Misses        436      436           

Impacted Files	Coverage Δ
packages/rum/src/domain/record/mutationObserver.ts	96.37% <ø> (ø)
packages/rum/src/domain/record/observers.ts	73.65% <ø> (ø)
packages/rum/src/domain/record/record.ts	70.96% <ø> (ø)
packages/rum/test/htmlAst.ts	91.66% <ø> (ø)
packages/rum/src/domain/record/viewports.ts	75.00% <40.00%> (+1.47%)	⬆️
packages/rum/src/domain/record/serialize.ts	91.02% <100.00%> (+0.05%)	⬆️

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[liywjl]

Nice work!