Fix a11y root element having 0×0 dimensions on web

compose/ui/ui/src/webMain/kotlin/androidx/compose/ui/window/ComposeWindow.web.kt

@@ -139,6 +139,8 @@ fun ComposeViewport(
                 position = "absolute"
                 top = "0"
                 left = "0"
+                width = "100%"
+                height = "100%"
             }
             appContainer.appendChild(a11yContainer)
         }

compose/ui/ui/src/webMain/kotlin/androidx/compose/ui/window/ComposeWindowInternal.web.kt

@@ -501,6 +501,10 @@ internal class ComposeWindow(
         canvas.style.width = "${boxSize.width.value}px"
         canvas.style.height = "${boxSize.height.value}px"
 
+        // The a11y container is sized via CSS (`width: 100%; height: 100%`) at construction
+        // time, so it tracks the canvas automatically without a per-resize bridge call across
+        // the wasm2js boundary. See ComposeViewport for the setup.
+
         _windowInfo.containerSize = sizeInPx
         _windowInfo.containerDpSize = boxSize
 

compose/ui/ui/src/webTest/kotlin/androidx/compose/ui/platform/a11y/A11yContainerSizingTest.kt

@@ -0,0 +1,59 @@
+/*
+ * Copyright 2026 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package androidx.compose.ui.platform.a11y
+
+import androidx.compose.material.Text
+import androidx.compose.ui.OnCanvasTests
+import kotlin.test.Test
+import kotlin.test.assertNotNull
+import kotlin.test.assertTrue
+import kotlinx.coroutines.test.runTest
+
+/**
+ * Regression test for https://youtrack.jetbrains.com/issue/CMP-10172.
+ *
+ * The a11y root container (`cmp_a11y_root`) was created with `position: absolute` but never
+ * given a width or height, leaving it as a 0×0 element in the DOM. Because hit-test-based
+ * accessibility tools (Apple Accessibility Inspector, Appium) walk down from the parent's
+ * bounding rect, they could not reach any Compose semantic node. VoiceOver was unaffected
+ * because it traverses the DOM tree sequentially, which masked the bug.
+ */
+class A11yContainerSizingTest : OnCanvasTests {
+
+    @Test
+    fun a11yContainerHasNonZeroRenderedSizeAfterInit() = runTest {
+        createComposeWindow {
+            Text("a11y sizing regression")
+        }
+
+        val a11yContainer = assertNotNull(
+            getA11YContainer(),
+            "A11Y container must exist when isA11YEnabled is true (default)"
+        )
+
+        val rect = a11yContainer.getBoundingClientRect()
+
+        assertTrue(
+            rect.width > 0.0,
+            "a11y container rendered width must be non-zero, was ${rect.width}"
+        )
+        assertTrue(
+            rect.height > 0.0,
+            "a11y container rendered height must be non-zero, was ${rect.height}"
+        )
+    }
+}