Once this CL (https://chromium-review.googlesource.com/c/chromium/src/+/1094097) lands, overscroll will calculated and handled in PropertyTree::ScrollBy():
https://cs.chromium.org/chromium/src/cc/trees/property_tree.cc?rcl=3007d0de31f5029af4168f455c0641573d13011f&l=1684

ScrollBy() should not consume any scroll if there's overscroll. Instead, InputScrollElasticityController should handle the overscroll, before ScrollBy() does.

This was not implemented in the CL because it's not trivial to do that.
Without calling ScrollBy() first, the ScrollElasticityHelper is not set up properly for InputScrollElasticity to handle the overscroll first.