Add a demo for touch events issues on web (#2521)

Related issue https://youtrack.jetbrains.com/issue/CMP-9030


## Testing
N/A

## Release Notes
N/A

Author: eymar

compose/mpp/demo/src/webMain/kotlin/androidx/compose/mpp/demo/bugs/BugsScreen.kt

@@ -21,5 +21,10 @@ import androidx.compose.mpp.demo.Screen
 val BugsScreen = Screen.Selection("Web Bug Reproducers", screens = listOf(
     Screen.Example("Text Fields in Scrollable") {
         TextFieldsInTallScrollableContainer()
+    },
+    Screen.Example("Touch events") {
+        // https://youtrack.jetbrains.com/issue/CMP-9030/wasm-canvas-incorrect-handling-of-multi-touch-input
+        PointerInputDebugOverlay()
     }
-))
+))
+

compose/mpp/demo/src/webMain/kotlin/androidx/compose/mpp/demo/bugs/PointerInputDebugOverlay.kt

@@ -0,0 +1,96 @@
+/*
+ * Copyright 2025 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.mpp.demo.bugs
+
+import androidx.compose.foundation.Canvas
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.BoxScope
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.material.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.geometry.Offset
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.input.pointer.pointerInput
+import androidx.compose.ui.unit.Dp
+import androidx.compose.ui.unit.dp
+
+// https://youtrack.jetbrains.com/issue/CMP-9030/wasm-canvas-incorrect-handling-of-multi-touch-input
+@Composable
+fun PointerInputDebugOverlay(
+    modifier: Modifier = Modifier.fillMaxSize(),
+    circleRadius: Dp = 50.dp,
+    circleColor: Color = Color.Red,
+) {
+    // Keep a map of active pointer positions (keyed by pointer id value)
+    val pointers = remember { mutableStateOf<Map<Long, Offset>>(emptyMap()) }
+    val log = remember { mutableStateOf("") }
+
+    fun logString(s: String) {
+        log.value = s + "\n" + log.value
+        if (log.value.length > 500) {
+            log.value = log.value.take(500)
+        }
+    }
+
+    Box(modifier = modifier) {
+        Text(log.value)
+        // overlay that listens to pointer events and draws circles
+        Box(
+            modifier = Modifier
+                .matchParentSize()
+                .pointerInput(Unit) {
+                    // Use awaitPointerEventScope to get raw pointer events and positions
+                    awaitPointerEventScope {
+                        while (true) {
+                            val event = awaitPointerEvent()
+                            logString(">")
+
+                            // update positions for each pointer change
+                            for (change in event.changes) {
+                                val idValue = change.id.value
+                                if (change.pressed) {
+                                    // pointer is down or moved
+                                    pointers.value += (idValue to change.position)
+                                    logString("+ $idValue")
+                                } else {
+                                    // pointer was released
+                                    logString("- $idValue")
+                                    pointers.value -= idValue
+                                }
+                            }
+                        }
+                    }
+                }
+        ) {
+            // Draw the active pointer locations
+            Canvas(modifier = Modifier.matchParentSize()) {
+                val radiusPx = circleRadius.toPx()
+                for ((_, pos) in pointers.value) {
+                    // draw a small red circle at each pointer position
+                    drawCircle(
+                        color = circleColor,
+                        radius = radiusPx,
+                        center = pos
+                    )
+                }
+            }
+        }
+    }
+}