Specify expected behavior for overflows and containment about layout-instability

[skobes]

Generally speaking, if a descendant is visible, it will be fully enclosed by its ancestor. Therefore, if ancestor and descendant shift together, there is no additional contribution to the impact region from the descendant.

Did you mean something different? Perhaps you could share a test case?

[wangxianzhu]

I found I misunderstood the "box" concept. The "principle box" is defined as to contain its descendant boxes and generated content. Then the spec is precise about ancestors and descendants.

[wangxianzhu]

Note that currently Chromium doesn't conform to the spec about visible overflows from descendants.

[wangxianzhu]

I'm reopening this issue because there is still things to be clarified.

<div style="width: 1000px; height: 10px">
  <div stye="width: 10px; height: 1000px"></div>
</div>

When the parent moves, currently Chrome reports 1000x1000 area is moved, but the real covered area is 1000x10 + 10x990. For implementation, the former is more performance and complexity friendly.