Bug 292516 – Inserting a subgrid child with `position: absolute` can cause `auto` column widths to be calculated incorrectly.

• Status: NEW
• Component: Layout and Rendering

Overview:

Inserting a subgrid child with `position: absolute` can cause `auto` column widths to be calculated incorrectly.

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Reproduction:

1. Go to https://codesandbox.io/p/sandbox/sleepy-noyce-vynd7s
2. Hover over the subgrid element to inject an element.
3. Or click the 'toggle inserted element' button.

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Generic Steps to Reproduce:

1. Create a grid with an `auto` width column and at least one other non-`auto` width column.

2. Create a subgrid that spans the entire parent grid.

3. Have a child that is always visible as part of the subgrid in the `auto` column.

4. Insert an absolutely positioned child in the non-`auto` column, after the initial load.

5. (Optional) remove the second child, the bug remains.

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Actual Results:

The `auto` width column has an incorrect width. It looks like it is being treated as `1fr` instead.

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Expected Results:

The `auto` width column should not be affected by the addition of a sibling with `position: absolute`.

The initial column width, before inserting the absolutely positioned second child, is correct and consistent with other browsers.

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Build Date & Hardware:

Safari 18.3 (19620.2.4.111.8, 19620) on MacOS 14.7.2

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Additional Builds and Platforms:

Not reproducible on Chrome or Firefox.

[Karl Dubost]

This is interesting because sometimes I get the right layout. After a while the dashed border around subgrid child is exhibiting the right width. it takes a couple of seconds.

[Radar WebKit Bug Importer]

<rdar://problem/151125072>