RenderIntrinsicWidth documentation is slightly misleading.

[tvolkert]

Currently, if RenderIntrinsicWidth is given tight constraints, it won't even ask its child for its intrinsic width - it'll just give the child the same constraints:

flutter/packages/flutter/lib/src/rendering/proxy_box.dart

Lines 641 to 646 in 77efc00

	BoxConstraints childConstraints = constraints;
	if (!childConstraints.hasTightWidth) {
	final double width = child.getMaxIntrinsicWidth(childConstraints.maxHeight);
	assert(width.isFinite);
	childConstraints = childConstraints.tighten(width: _applyStep(width, _stepWidth));
	}

This seems wrong, and in contradiction to the documentation on the class:

Sizes its child's width to the child's maximum intrinsic width ... This class is useful, for example, when unlimited width is available and you would like a child that would otherwise attempt to expand infinitely to instead size itself to a more reasonable width.

It seems like, much like RenderFlex, RenderIntrinsicWidth could honor its constraints while allowing its child to be smaller than the parent (RenderIntrinsicWidth).

For example, consider the following Render Tree:

RenderView
│ configuration: Size(800, 600) at 1.0x (in logical pixels)
│ size: Size(800, 600)
└ RenderIntrinsicWidth
  │ constraints: BoxConstraints(w=800, h=600)
  │ size: Size(800, 600)
  └ RenderConstrainedBox
    │ constraints: BoxConstraints(w=800, h=600)
    │ additionalConstraints: BoxConstraints(w=200, h=200)
    │ size: Size(800, 600)

In this case, the RenderIntrinsicWidth needs to be 800x600 because it received tight constraints by its parent RenderView. However, it should relax those constraints that it passes to the child when it asks for the child's max intrinsic width to be:

childConstraints: BoxConstraints(0.0<=w<=800.0, h=600.0)

This would yield the following tree, which would be truer to the documented behavior of the render object:

RenderView
│ configuration: Size(800, 600) at 1.0x (in logical pixels)
│ size: Size(800, 600)
└ RenderIntrinsicWidth
  │ constraints: BoxConstraints(w=800, h=600)
  │ size: Size(800, 600)
  └ RenderConstrainedBox
    │ constraints: BoxConstraints(w=200, h=600) // because they're tightened to the child's intrinsic width
    │ additionalConstraints: BoxConstraints(w=200, h=200)
    │ size: Size(200, 600)

Additional information

All of the above also applies to RenderIntrinsicHeight

[tvolkert]

After discussing with @goderbauer, this is working as intended - the caller can use Align (or Row/Column) to satisfy the use case described in this issue.

Re-purposing this issue to cover making the documentation clearer.

[github-actions]

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