[UncertaintyDeterminesYou4ndMe]

Description

An element with a maximum size along the axis it scrolls does not scroll: under v_flex().max_h(px(320.)).overflow_y_scrollbar(), the rows past 320px are clipped and cannot be reached. Both copies of Scrollable, in gpui-component and in gpui-shell, have the code involved; this changes both.

Scrollable wraps the element. The wrapper copies its size, minimum size and maximum size, and the element becomes the content of a scroll area inside the wrapper. Two things keep such an element from scrolling:

• The content keeps the maximum size as well, so it is never larger than the area and nothing overflows. The content now drops the maximum size on the scrolled axis; the wrapper still applies it.
• With GPUI Fast, the area's size_full(), 100% of the wrapper, does not hold it to the wrapper's maximum size. In a column whose height follows its content, placed in a row, the area comes out as tall as the inside of that column (140px for a 100px maximum between two 20px blocks), and the wrapper clips it. With the default GPUI the area already gets the wrapper's 100px; the two use different Taffy versions (0.14 and 0.13). The wrapper is now a flex container along the scrolled axis, so the area is a flex item and shrinks to the wrapper's size with either. Where the wrapper's size is fixed or comes from its parent, as with flex_1() or size_full(), the area keeps the size it had.

ScrollBounce takes the layout of the area it wraps, and ScrollableMask is absolutely positioned, so neither becomes a second flex item. With both axes (overflow_scrollbar()) the wrapper is a column, so only the area's height is held to the wrapper's maximum this way.

Found in a form whose fields sit under max_h(320).overflow_y_scrollbar() inside a card.

How to Test

cargo test -p gpui-component --lib -- scroll::scrollable
cargo test -p gpui-component --features gpui-fast,test-support --lib -- scroll::scrollable
cargo test -p gpui-shell --release --lib -- scroll::tests

max_height_scrollable_scrolls_to_its_last_row (gpui-component) and scrollable_with_a_max_height_scrolls_what_overflows_it (gpui-shell) lay out the column described above and scroll it to its last row. max_width_scrollable_scrolls_what_overflows_it and scrollable_with_a_max_width_scrolls_what_overflows_it scroll a row with a maximum width. Without the first change all four fail. Without the flex wrapper the two max-height tests fail with GPUI Fast (gpui-shell uses it by default): the last row ends 140px below the first row's top instead of 100px. With the default GPUI they pass either way. The gpui-shell flex-item test now also scrolls to its last row; it passes before and after the change.

AI-assisted: written with Claude Code and reviewed by me before submission.

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (If any) is accurate.
• Passed cargo run for story tests related to the changes. (Not run; the tests above cover the layout.)
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific). (Not platform-specific.)