diff --git a/platform/platform-impl/src/com/intellij/ide/impl/ProjectUtil.kt b/platform/platform-impl/src/com/intellij/ide/impl/ProjectUtil.kt
index f9844bdd8f85..ca77dda34559 100644
--- a/platform/platform-impl/src/com/intellij/ide/impl/ProjectUtil.kt
+++ b/platform/platform-impl/src/com/intellij/ide/impl/ProjectUtil.kt
@@ -188,11 +188,15 @@ object ProjectUtil {
       }
     }
 
-    val storePathManager = serviceAsync<ProjectStorePathManager>()
-    val descriptor = withContext(Dispatchers.IO) {
-      storePathManager.getStoreDescriptor(file)
-    }
-    if (descriptor.testStoreDirectoryExistsForProjectRoot()) {
+    // `isDirectory` test here is for backward compatibility with 252: in 252 we never entered this method with regular files - only with
+    // directories. The problem here is that any regular file now has a storeDescriptor (getStoreDescriptor return type is not nullable),
+    // which evaluates to `regularFile.parent.resolve(".idea")`, which, if exists, pushes IDE to open a new project, instead of a file in
+    // the opened project. This is a tiny quick-fix for 253. We should rework the project open flow to avoid this strange check here.
+    if (Files.isDirectory(file) && isValidProjectPath(file)) {
+      val descriptor = withContext(Dispatchers.IO) {
+        serviceAsync<ProjectStorePathManager>().getStoreDescriptor(file)
+      }
+
       LOG.info("Opening existing project with .idea at $file")
       // see OpenProjectTest.`open valid existing project dir with inability to attach using OpenFileAction` test about why `runConfigurators = true` is specified here
       val options = options.copy(
diff --git a/platform/platform-tests/testSrc/com/intellij/openapi/project/OpenProjectTest.kt b/platform/platform-tests/testSrc/com/intellij/openapi/project/OpenProjectTest.kt
index 3cdc6ba804be..107ec91e322e 100644
--- a/platform/platform-tests/testSrc/com/intellij/openapi/project/OpenProjectTest.kt
+++ b/platform/platform-tests/testSrc/com/intellij/openapi/project/OpenProjectTest.kt
@@ -6,6 +6,7 @@ import com.intellij.ide.impl.ProjectUtil
 import com.intellij.ide.impl.ProjectUtil.FolderOpeningMode.AS_FOLDER
 import com.intellij.ide.impl.ProjectUtil.FolderOpeningMode.AS_PROJECT
 import com.intellij.ide.impl.SelectProjectOpenProcessorDialog
+import com.intellij.openapi.project.TestOpenMode.ModeFileOrFolderDefault
 import com.intellij.openapi.project.TestOpenMode.ModeFolderAsFolder
 import com.intellij.openapi.project.TestOpenMode.ModeFolderAsProject
 import com.intellij.openapi.project.TestProjectSource.SourceCLI
@@ -41,15 +42,15 @@ import kotlin.io.path.writeText
 // clean: .idea doesn't exists
 // existing: project directory exists
 // nested: .idea exists and ../.idea exists too
-// multibuild: does not exist, and there are 2 marker build files (pom.xml and build.gradle)
-// TODO: open regular file
+// multibuild: .idea does not exist, and there are 2 marker build files (pom.xml and build.gradle)
+// regular file: regular file that is not a folder
 
 // with ability to attach - there is some defined ProjectAttachProcessor extension (e.g. WS, PS).
 // with inability to attach - there is no any defined ProjectAttachProcessor extension (e.g. IU, IC).
 
 
 enum class TestProjectSource { SourceOpenFileAction, SourceCLI }
-enum class TestOpenMode { ModeFolderAsProject, ModeFolderAsFolder }
+enum class TestOpenMode { ModeFileOrFolderDefault, ModeFolderAsProject, ModeFolderAsFolder }
 
 @RunWith(Parameterized::class)
 internal class OpenProjectTest(private val opener: Opener) {
@@ -79,7 +80,7 @@ internal class OpenProjectTest(private val opener: Opener) {
         // I don't have strong opinion about defaultProjectTemplateShouldBeAppliedOverride.
         // Weak opinion: a folder is not a project => we don't need default project settings.
         // Feel free to change the test if you have strong opinion about desired behavior.
-        Opener(SourceCLI, ModeFolderAsFolder, expectedModules = emptyList(), expectedRoots = listOf($$"$ROOT$"), defaultProjectTemplateShouldBeAppliedOverride = false) {
+        Opener(SourceCLI, ModeFileOrFolderDefault, expectedModules = emptyList(), expectedRoots = listOf($$"$ROOT$"), defaultProjectTemplateShouldBeAppliedOverride = false) {
           runBlocking { CommandLineProcessor.doOpenFileOrProject(it, createOrOpenExistingProject = false, false) }.project!!
         },
       )
@@ -152,9 +153,9 @@ internal class OpenProjectTest(private val opener: Opener) {
   @Test
   fun `open multibuild existing project dir with inability to attach`() = runBlocking(Dispatchers.Default) {
     Assume.assumeTrue(
-      "This test does not handle ProjectAsProject mode yet, because `null` from SelectProjectOpenProcessorDialog" +
+      "This test does not handle ModeFolderAsProject mode yet, because `null` from SelectProjectOpenProcessorDialog" +
       " has different behavior when opening folder from CLI and from open action, and we don't want to cement this behavior in tests.",
-      opener.mode == ModeFolderAsFolder,
+      opener.mode != ModeFolderAsProject,
     )
 
     val processorNames = ProjectOpenProcessor.EXTENSION_POINT_NAME.extensionList.map(ProjectOpenProcessor::name)
@@ -171,6 +172,41 @@ internal class OpenProjectTest(private val opener: Opener) {
     }
   }
 
+  @Test
+  fun `open project then open regular file in the same valid existing project dir with inability to attach`() = runBlocking(Dispatchers.Default) {
+    Assume.assumeTrue(
+      "Ignore ModeFolderAsProject/ModeFolderAsFolder, because we are checking open of regular files here, not folders",
+      opener.mode != ModeFolderAsProject && opener.mode != ModeFolderAsFolder,
+    )
+
+    val projectDir = tempDir.newPath("project")
+    projectDir.resolve(".idea").createDirectories()
+    val javaFile = projectDir.resolve("MyClass.java")
+    javaFile.writeText("public class MyClass {}")
+
+    ExtensionTestUtil.maskExtensions(ProjectAttachProcessor.EP_NAME, listOf(), disposableRule.disposable)
+    opener.opener(projectDir)!!.useProject { openedProject ->
+      val project = opener.opener(javaFile)
+      // the file should be opened in the already opened project
+      assertThat(project).isSameAs(openedProject)
+    }
+    Unit
+  }
+
+  @Test
+  fun `open project then open the the same valid existing project dir with inability to attach`() = runBlocking(Dispatchers.Default) {
+    val projectDir = tempDir.newPath("project")
+    projectDir.resolve(".idea").createDirectories()
+
+    ExtensionTestUtil.maskExtensions(ProjectAttachProcessor.EP_NAME, listOf(), disposableRule.disposable)
+    opener.opener(projectDir)!!.useProject { openedProject ->
+      val project = opener.opener(projectDir)
+      // this should bring already opened project to foreground
+      assertThat(project).isSameAs(openedProject)
+    }
+    Unit
+  }
+
   private fun setupMultibuildProject(): Path {
     val projectDir = tempDir.newPath("project")
     projectDir.createDirectories()