Let users enter their full Matrix user ID when searching for a homeserver

The account provider search treated its whole input as a host, so a full user
ID was probed as https://@alice:example.org, which never resolves, and the
permissive URL fallback then offered that invalid address as a provider.

Recognise a complete user ID and resolve the homeserver from its server name,
preserving a port when there is one. Input that starts with @ but is not yet a
complete user ID resolves nothing, so no candidate is produced while the user
is still typing. Detection reuses MatrixPatterns.isUserId and UserId.domainName
rather than a new pattern.

features/login/impl/src/main/kotlin/io/element/android/features/login/impl/resolver/HomeserverResolver.kt

@@ -14,6 +14,8 @@ import io.element.android.libraries.core.coroutine.parallelMap
 import io.element.android.libraries.core.uri.ensureProtocol
 import io.element.android.libraries.core.uri.isValidUrl
 import io.element.android.libraries.matrix.api.auth.HomeServerLoginCompatibilityChecker
+import io.element.android.libraries.matrix.api.core.MatrixPatterns
+import io.element.android.libraries.matrix.api.core.UserId
 import io.element.android.libraries.permissions.api.localnetwork.LocalNetworkPermissionAdvisor
 import kotlinx.coroutines.currentCoroutineContext
 import kotlinx.coroutines.flow.Flow
@@ -34,8 +36,13 @@ class HomeserverResolver(
     fun resolve(userInput: String): Flow<List<HomeserverData>> = flow {
         val flowContext = currentCoroutineContext()
         val trimmedUserInput = userInput.trim()
-        if (trimmedUserInput.length < 4) return@flow
-        val candidateBase = trimmedUserInput.ensureProtocol().removeSuffix("/")
+        val searchTerm = when {
+            MatrixPatterns.isUserId(trimmedUserInput) -> UserId(trimmedUserInput).domainName.orEmpty()
+            trimmedUserInput.startsWith("@") -> return@flow
+            else -> trimmedUserInput
+        }
+        if (searchTerm.length < 4) return@flow
+        val candidateBase = searchTerm.ensureProtocol().removeSuffix("/")
         val list = getUrlCandidates(candidateBase)
         val currentList = Collections.synchronizedList(mutableListOf<HomeserverData>())
         // Run all the requests in parallel

features/login/impl/src/test/kotlin/io/element/android/features/login/impl/resolver/HomeserverResolverTest.kt

@@ -0,0 +1,108 @@
+/*
+ * Copyright (c) 2026 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+package io.element.android.features.login.impl.resolver
+
+import app.cash.turbine.test
+import com.google.common.truth.Truth.assertThat
+import io.element.android.libraries.matrix.test.auth.FakeHomeServerLoginCompatibilityChecker
+import io.element.android.libraries.permissions.test.FakeLocalNetworkPermissionAdvisor
+import io.element.android.tests.testutils.lambda.lambdaRecorder
+import io.element.android.tests.testutils.lambda.value
+import io.element.android.tests.testutils.testCoroutineDispatchers
+import kotlinx.coroutines.test.TestScope
+import kotlinx.coroutines.test.runTest
+import org.junit.Test
+
+class HomeserverResolverTest {
+    @Test
+    fun `resolve - a server name is used as is`() = runTest {
+        val checkResult = lambdaRecorder<String, Result<Boolean>> { Result.success(it == "https://example.org") }
+        val sut = createHomeserverResolver(checkResult)
+        sut.resolve("example.org").test {
+            assertThat(awaitItem()).isEqualTo(listOf(HomeserverData(homeserverUrl = "https://example.org")))
+            awaitComplete()
+        }
+        checkResult.assertions().isCalledExactly(1)
+            .withSequence(
+                listOf(value("https://example.org")),
+            )
+    }
+
+    @Test
+    fun `resolve - an url is used as is`() = runTest {
+        val checkResult = lambdaRecorder<String, Result<Boolean>> { Result.success(it == "https://example.org") }
+        val sut = createHomeserverResolver(checkResult)
+        sut.resolve("https://example.org").test {
+            assertThat(awaitItem()).isEqualTo(listOf(HomeserverData(homeserverUrl = "https://example.org")))
+            awaitComplete()
+        }
+        checkResult.assertions().isCalledExactly(1)
+            .withSequence(
+                listOf(value("https://example.org")),
+            )
+    }
+
+    @Test
+    fun `resolve - a user id resolves the server name of the user id`() = runTest {
+        val checkResult = lambdaRecorder<String, Result<Boolean>> { Result.success(it == "https://example.org") }
+        val sut = createHomeserverResolver(checkResult)
+        sut.resolve("@alice:example.org").test {
+            assertThat(awaitItem()).isEqualTo(listOf(HomeserverData(homeserverUrl = "https://example.org")))
+            awaitComplete()
+        }
+        checkResult.assertions().isCalledExactly(1)
+            .withSequence(
+                listOf(value("https://example.org")),
+            )
+    }
+
+    @Test
+    fun `resolve - a user id with a port resolves the server name of the user id`() = runTest {
+        val checkResult = lambdaRecorder<String, Result<Boolean>> { Result.success(it == "https://example.org:8448") }
+        val sut = createHomeserverResolver(checkResult)
+        sut.resolve("@alice:example.org:8448").test {
+            assertThat(awaitItem()).isEqualTo(listOf(HomeserverData(homeserverUrl = "https://example.org:8448")))
+            awaitComplete()
+        }
+        checkResult.assertions().isCalledExactly(1)
+            .withSequence(
+                listOf(value("https://example.org:8448")),
+            )
+    }
+
+    @Test
+    fun `resolve - an incomplete user id does not resolve anything`() = runTest {
+        val checkResult = lambdaRecorder<String, Result<Boolean>> { Result.success(true) }
+        val sut = createHomeserverResolver(checkResult)
+        sut.resolve("@alice").test {
+            awaitComplete()
+        }
+        sut.resolve("@alice:").test {
+            awaitComplete()
+        }
+        checkResult.assertions().isNeverCalled()
+    }
+
+    @Test
+    fun `resolve - a user id with a too short server name does not resolve anything`() = runTest {
+        val checkResult = lambdaRecorder<String, Result<Boolean>> { Result.success(true) }
+        val sut = createHomeserverResolver(checkResult)
+        sut.resolve("@alice:a.b").test {
+            awaitComplete()
+        }
+        checkResult.assertions().isNeverCalled()
+    }
+
+    private fun TestScope.createHomeserverResolver(
+        checkResult: (String) -> Result<Boolean>,
+    ) = HomeserverResolver(
+        dispatchers = testCoroutineDispatchers(),
+        homeServerLoginCompatibilityChecker = FakeHomeServerLoginCompatibilityChecker(checkResult = checkResult),
+        localNetworkPermissionAdvisor = FakeLocalNetworkPermissionAdvisor(),
+    )
+}