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FlowRow and FlowColumn wrap children onto new lines when they run out of room. They replace the custom Layout most codebases wrote for chip groups, and the overflow API handles the 'and 4 more' case.
HorizontalPager brings paging into Compose with no external dependency. The interesting surface is PagerState — currentPage versus settledPage, scroll versus animateScrollToPage, and how to sync tabs without a loop.
LazyVerticalGrid adds cell strategy and span control on top of lazy lists. GridCells.Adaptive removes most breakpoint code, and span sizing is what lets headers live in the same grid as items.
LazyColumn composes only visible items and reuses slots as you scroll. Without a stable key that reuse attaches state to the wrong rows — the same positional-identity bug as Day 8, at scale.
Most custom modifiers should be plain extension functions returning a chain. Modifier.Node is for elements that need their own state or draw/measure participation, and it replaced composed { } for good reasons.
Compose sizing confuses people because Constraints is a min/max range rather than a value. Each sizing modifier transforms the range in a specific way, and reading them that way makes layout deterministic.
Modifier order changes rendered output because each element wraps the remainder of the chain. The outside-in reading rule resolves padding, background, clickable ripple bounds and clip behaviour in one go.
Modifier looks like a fluent builder but is an ordered immutable linked list. Knowing its actual shape explains chaining, reuse, the modifier parameter convention, and why some combinations do nothing.
Compose's layout system measures every child exactly once. Understanding that single-pass guarantee explains Column, Row and Box, and why some things you could do in View layouts are deliberately impossible.
Compose has two kinds of state holder: plain classes that live in the composition, and ViewModels that outlive it. Knowing which to reach for is what keeps UiState classes from growing to twenty fields.
Compose's recommended architecture is three layers with one-directional dependencies. The split inside the UI layer — elements and state holders — is the one that decides whether a screen can be tested without a device.
CompositionLocal passes values down the composition without parameters. That invisibility is the feature and the danger: three tests for when it is the right tool, and what to use instead when it isn't.
