contain-intrinsic-size

Per [1],

These properties allow elements with size containment to specify an explicit intrinsic inner size, causing the box to size as if its in-flow content totals to a width and height matching the specified explicit intrinsic inner size (rather than sizing as if it were empty).

The implementation is roughly split into two parts:

• Get the size from the contain-intrinsic-* properties as an explicit intrinsic inner size.
• Make the sizecontainment box sizing as if its content is the explicit intrinsic inner size.

The explicit intrinsic inner size

The part has landed in [2].

• Added explicitIntrinsicInnerLogicalWidth/Height() to RenderBox.h, it gets the value of contain-intrinsic-* from RenderStyle.
• To handle the logicalWidth, in computeIntrinsicLogicalWidths, the intrinsic size of sizeContainment was zero, now it is explicitIntrinsicInnerLogicalWidth() if existing. We need to handle every computeIntrinsicLogicalWidths() in the code.
• To handle the logicalWidth, in updateLogicalHeight(), sizeContainment was treated as if there is no content, now it added explicitIntrinsicInnerLogicalHeight() if existing.

The size from the contain-intrinsic-* properties

Value none and value length

These two values are simple, for none, there is nothing to do, for length, we need to set the value to RenderStyle.

Value auto length

The explicit intrinsic inner size is its last remembered size if any, if no, then it is the value of length.

The last remembered size

Per [3], the last remembered size is the size when the element is not size-contained. The value is determined when ResizeObserver events are determined and delivered.

• The last remembered size is related to Element, we add RefPtr<ResizeObserverSize> m_lastRememberedSize to ElementRareData. See the implementation in [4].
• Create a ResizeObserver to handle the last remembered size. See the implementation in [5].
  • To create a specific ResizeObserver, we need to create a callback to handle the last remembered size, it is CallbackForContainIntrinsicSize, which removes or saves LastRememberedSize to the target.
  • Document will create and own m_resizeObserverForContainIntrinsicSize if there is an element with contain-intrinsic-size: auto length.
  • In RenderTreeUpdater::updateElementRenderer, it will observe the element if it is with contain-intrinsic-size: auto length, if not unobserve it.
  • When deliverResizeObservations, the LastRememberedSize will be saved.

[1] ​https://w3c.github.io/csswg-drafts/css-sizing-4/#intrinsic-size-override
[2] ​https://github.com/WebKit/WebKit/pull/1799
[3] ​https://w3c.github.io/csswg-drafts/css-sizing-4/#last-remembered
[4] ​https://github.com/WebKit/WebKit/pull/7375
[5] ​https://github.com/WebKit/WebKit/pull/7829