The main thread generates new viewports around the impl thread scroll each main frame.

sunnyps: did this get solved?

vmiura@ investigated the reasons why we perform commits during compositor thread scrolling.  Paraphrasing from offline discussion we just had:
1) Non root layer scrolling always calls SetNeedsCompositingInputsUpdate
2) Fixed position elements above a scrolling layer cause lots of invalidations
3) It seems necessary to set will-change: transform in addition to overflow: scroll to get composited scrolling

https://cs.chromium.org/chromium/src/third_party/blink/renderer/core/paint/paint_layer_scrollable_area.cc?q=paintlayerscroll&sq=package:chromium&g=0&l=2170

In addition to these issues, Scrollbar always calls SetNeedsDisplay on the scrollbar layer irrespective of what parts of the scrollbar change:

https://cs.chromium.org/chromium/src/third_party/blink/renderer/core/scroll/scrollbar.cc?rcl=173a5dada833bc96df1dcf54cd0bd547fa1e3d18&l=652

There are two parts to this problem:
1) Whether BeginMainFrame happens at all
2) How much work does BeginMainFrame do? Paint invalidation, commit, etc.