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