size

size() provides data to change the size of the floating element. For instance, you can resize it so it doesn't overflow the available space:

If your floating element's content cannot be resized such as in the example, you can make the floating element scrollable with overflow: scroll (or auto).

Usage

import {computePosition, size} from '@floating-ui/dom';
 
computePosition(referenceEl, floatingEl, {
  middleware: [
    size({
      apply({width, height, reference, floating}) {
        // Do things with the data, e.g.
        Object.assign(floatingEl.style, {
          maxWidth: `${width}px`,
          maxHeight: `${height}px`,
        });
      },
    }),
  ],
});

If your element has a border, ensure your CSS is using box-sizing: border-box.

Options

These are the options you can pass to size().

interface Options extends DetectOverflowOptions {
  apply?: (args: Dimensions & ElementRects) => void;
}

apply

default: undefined

Unlike other middleware, in which you assign styles after computePosition() has done its work, size() has its own apply function to do the work during the lifecycle:

size({
  apply({width, height}) {
    // Style mutations here
  },
});

This is because the x and y coordinates become incorrect after you've changed the size, and so changes to the dimensions of the floating element need to be performed before the coordinates get assigned to it.

...detectOverflowOptions

All of detectOverflow's options can be passed. For instance:

size({padding: 5}); // 0 by default

Using with flip

Using size() together with flip() enables some useful behavior. The floating element can be resized, thus allowing it to prefer its initial placement as much as possible, until it reaches a minimum size, at which point it will flip.

If you're using the padding option in either middleware, ensure they share the same value.

bestFit

The 'bestFit' fallback strategy in the flip() middleware is the default, which ensures the best fitting placement is used. In this scenario, place size() after flip():

const middleware = [
  flip(),
  size({
    apply({width, height}) {
      // ...
    },
  }),
];

This strategy ensures the floating element stays in view at all times at the most optimal size.

initialPlacement

If instead, you want the initial placement to take precedence, and are setting a minimum acceptable size, place size() before flip():

const middleware = [
  size({
    apply({width, height}) {
      Object.assign(floatingEl.style, {
        // Minimum acceptable height is 50px.
        // `flip` will then take over.
        maxHeight: `${Math.max(50, height)}px`,
      });
    },
  }),
  flip({
    fallbackStrategy: 'initialPlacement',
  }),
];

Match reference width

A common feature of select dropdowns is that the dropdown matches the width of the reference regardless of its contents. You can also use size() for this, as the Rects get passed in:

size({
  apply({reference}) {
    Object.assign(floatingEl.style, {
      width: `${reference.width}px`,
    });
  },
});