`@container (width > 10em)` never matches: a container query's `em` is never resolved

Start with src/native/conditions/container-query.ts and the named container width and height families in src/native/reactivity.ts; compare how src/native/styles/units.ts reads an element's em value. Run the reproducer with yarn jest src/__tests__/native/container-em-bound.test.tsx (the test file is on the reproducer branch). Done means all three reported conditions match the expected results using the container's computed font size.

Before submitting a new issue

• I tested using the latest version of the library, as the bug might be already fixed.
• I tested using a supported version of react native.
• I checked for possible duplicate issues, with possible answers.

Bug summary

In a container query condition, em is relative to the query container's computed font size (CSS Conditional 5 §6.1): in a 400px container that sets none, 10em is 160px, and Chromium 156, Firefox 157 and WebKit 27.2 match @container (width > 10em). The compiler emits the bound as [{}, "em", 10, 1], and testContainerMediaCondition (src/native/conditions/container-query.ts) compares the container's width only with a number, so the condition is false at every size.

not (width > 30em) matches on main only because the bound reads as false; #426 answers it unknown (MQ5 §3.1) and lists this among its known limits. #494 resolves a media query's em, which is the root font size rather than a container's.

Resolving it means measuring each container's computed fontSize beside its width and height (containerWidthFamily and containerHeightFamily in src/native/reactivity.ts), as the style resolver reads an element's em from __rn-css-em (src/native/styles/units.ts).

Library version

main at 2421ea7 (0.0.0-nightly.2421ea7)

Environment info

System:
  OS: Windows 11 10.0.26300
  CPU: "(32) x64 AMD Ryzen 9 9950X3D 16-Core Processor          "
  Memory: 155.26 GB / 255.67 GB
Binaries:
  Node:
    version: 26.12.1
    path: C:\Program Files\nodejs\node.EXE
  Yarn:
    version: 4.9.2
    path: C:\Users\<user>\AppData\Roaming\npm\yarn.CMD
  npm:
    version: 12.2.1
    path: C:\Program Files\nodejs\npm.CMD
  Watchman: Not Found
SDKs:
  Android SDK:
    API Levels:
      - "33"
      - "35"
      - "35"
      - "35"
      - "36"
      - "37"
    Build Tools:
      - 35.0.0
      - 36.0.0
      - 36.1.0
      - 37.0.0
    System Images:
      - android-26 | Google APIs Intel x86_64 Atom
      - android-28 | Google APIs Intel x86_64 Atom
      - android-29 | Google APIs Intel x86_64 Atom
      - android-30 | Google APIs Intel x86_64 Atom
      - android-31 | Google APIs Intel x86_64 Atom
      - android-32 | Google APIs Intel x86_64 Atom
      - android-33 | Google APIs Intel x86_64 Atom
      - android-34 | Google APIs Intel x86_64 Atom
      - android-35 | Google APIs Intel x86_64 Atom
      - android-36.1 | Google Play Intel x86_64 Atom
      - android-36.1 | Pre-Release 16 KB Page Size Google Play Intel x86_64 Atom
      - android-37.0 | Google APIs Intel x86_64 Atom
    Android NDK: Not Found
  Windows SDK: Not Found
IDEs:
  Android Studio: AI-262.9437.185.2621.16467767
  Visual Studio: Not Found
Languages:
  Java:
    version: 17.0.20.1
    path: C:\Program Files\Microsoft\jdk-17.0.20.101-hotspot\bin\javac.EXE
  Ruby: Not Found
npmPackages:
  "@react-native-community/cli": Not Found
  react:
    installed: 19.2.3
    wanted: 19.2.3
  react-native:
    installed: 0.86.3
    wanted: 0.86.3
  react-native-windows: Not Found
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: Not found
  newArchEnabled: Not found
iOS:
  hermesEnabled: Not found
  newArchEnabled: Not found

Steps to reproduce

1. Clone the reproducer branch, which is main at 2421ea7 with one file added, src/__tests__/native/container-em-bound.test.tsx, and install:

   git clone --branch repro/512-container-em-bound https://github.com/YevheniiKotyrlo/react-native-css
   cd react-native-css
   yarn install
   

2. Run yarn jest src/__tests__/native/container-em-bound.test.tsx.

src/__tests__/native/container-em-bound.test.tsx:

// A container query's em is the container's font size (CSS Conditional 5 §6.1), so 10em is 160px in a container that sets none
import { fireEvent, render, screen } from "@testing-library/react-native";
import { View } from "react-native-css/components/View";
import { registerCSS } from "react-native-css/jest";

test.each([
  ["(width > 10em)", true],
  ["(width > 30em)", false],
  ["not (width > 30em)", true],
])("@container %s in a 400px container matches: %s", (condition, expected) => {
  registerCSS(`
    .parent { container-type: size; }
    .child { width: 10px; }
    @container ${condition} { .child { width: 37px; } }
  `);
  render(
    <View testID="parent" className="parent">
      <View testID="child" className="child" />
    </View>,
  );
  fireEvent(screen.getByTestId("parent"), "layout", {
    nativeEvent: { layout: { x: 0, y: 0, width: 400, height: 200 } },
  });
  expect(screen.getByTestId("child").props.style.width === 37).toBe(expected);
});

Expected: @container (width > 10em) matches in the 400px container, where 10em is 160px, and not (width > 30em) matches too, so the test's three cases pass.

Actual: the bound never compares, so (width > 10em) is false at every size and its case fails.

FAIL src/__tests__/native/container-em-bound.test.tsx
  × @container (width > 10em) in a 400px container matches: true (42 ms)
  √ @container (width > 30em) in a 400px container matches: false (5 ms)
  √ @container not (width > 30em) in a 400px container matches: true (2 ms)
  ● @container (width > 10em) in a 400px container matches: true
    expect(received).toBe(expected) // Object.is equality
    Expected: true
    Received: false
      22 |     nativeEvent: { layout: { x: 0, y: 0, width: 400, height: 200 } },
      23 |   });
    > 24 |   expect(screen.getByTestId("child").props.style.width === 37).toBe(expected);
         |                                                                ^
      25 | });
      26 |
      at toBe (src/__tests__/native/container-em-bound.test.tsx:24:64)
Test Suites: 1 failed, 1 total
Tests:       1 failed, 2 passed, 3 total
Snapshots:   0 total
Time:        1.533 s, estimated 3 s
Ran all test suites matching /src\\__tests__\\native\\container-em-bound.test.tsx/i.

Reproducible example repository

https://github.com/YevheniiKotyrlo/react-native-css/tree/repro/512-container-em-bound