All 7 Layout Widgets (Row, Column, Stack, Container) questions
More in Layout Widgets (Row, Column, Stack, Container)
What are Row and Column, and how do they let you arrange several separate child widgets either horizontally or vertically alongside one another? What is the Container widget, and how does it let you conveniently combine padding, margin, background color, and border decoration all together within just one single genuinely convenient widget? How do Expanded and Flexible let you properly control exactly how much genuinely available space a specific given child should actually take up relative to its own siblings within a surrounding Row or Column? How do mainAxisAlignment and crossAxisAlignment properly control the specific alignment of children within a Row or Column along each of those particular widget's own two genuinely separate distinct axes? How does Flutter's own underlying constraint based layout system genuinely work, specifically regarding how constraints actually flow down the widget tree while sizes actually flow back up? How can you properly build a genuinely fully custom layout widget by directly extending MultiChildLayoutDelegate or CustomMultiChildLayout, for a genuinely specific complex arrangement not already covered by any of Flutter's own existing standard layout widgets?
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Flutter › Layout Widgets (Row, Column, Stack, Container)

How does the Stack widget let you genuinely layer several separate child widgets directly on top of one another, and how does the Positioned widget let you precisely control exactly where each individual layered child actually appears within that Stack?

Intermediate

Answer

Stack lets you place several separate child widgets so they genuinely overlap directly on top of one another rather than being arranged neatly side by side the way Row or Column would otherwise arrange them, with each subsequent child painted genuinely on top of the previous ones, and wrapping a specific given child with the Positioned widget lets you precisely and explicitly control exactly where that particular specific child should actually appear relative to the edges of that surrounding Stack, such as pinning a small badge icon to a specific product photo's own top right corner.
Stack(
  children: [
    Image.network(productImageUrl),
    Positioned(top: 8, right: 8, child: Badge(label: Text('New'))),
  ],
)
Real-world example A product card displays a small new badge precisely pinned to its own product image's top right corner using Stack together with Positioned, achieving a genuinely common, visually appealing layering effect that would simply not actually be possible using Row or Column alone.

Common follow-ups: What genuinely happens to a child within a Stack that is not actually wrapped with Positioned at all?;How do you actually properly center a specific child widget precisely directly within a surrounding Stack?

Custom Widgets & Reusable Components;Flutter Widgets Fundamentals (Stateless & Stateful)
All 7 Layout Widgets (Row, Column, Stack, Container) questions