Use correct default project for file if that project is opened at later time

Fixes #31926

Author: sheetalkamat

src/server/editorServices.ts

@@ -1533,6 +1533,14 @@ namespace ts.server {
             return undefined;
         }
 
+        /*@internal*/
+        findDefaultConfiguredProject(info: ScriptInfo) {
+            if (!info.isScriptOpen()) return undefined;
+            const configFileName = this.getConfigFileNameForFile(info);
+            return configFileName &&
+                this.findConfiguredProjectByProjectName(configFileName);
+        }
+
         /**
          * This function tries to search for a tsconfig.json for the given file.
          * This is different from the method the compiler uses because

src/server/scriptInfo.ts

@@ -491,21 +491,40 @@ namespace ts.server {
                 case 1:
                     return this.containingProjects[0];
                 default:
-                    // if this file belongs to multiple projects, the first configured project should be
-                    // the default project; if no configured projects, the first external project should
-                    // be the default project; otherwise the first inferred project should be the default.
+                    // If this file belongs to multiple projects, below is the order in which default project is used
+                    // - for open script info, its default configured project during opening is default if info is part of it
+                    // - first configured project of which script info is not a source of project reference redirect
+                    // - first configured project
+                    // - first external project
+                    // - first inferred project
                     let firstExternalProject;
                     let firstConfiguredProject;
-                    for (const project of this.containingProjects) {
+                    let firstNonSourceOfProjectReferenceRedirect;
+                    let defaultConfiguredProject: ConfiguredProject | false | undefined;
+                    for (let index = 0; index < this.containingProjects.length; index++) {
+                        const project = this.containingProjects[index];
                         if (project.projectKind === ProjectKind.Configured) {
-                            if (!project.isSourceOfProjectReferenceRedirect(this.fileName)) return project;
+                            if (!project.isSourceOfProjectReferenceRedirect(this.fileName)) {
+                                // If we havent found default configuredProject and
+                                // its not the last one, find it and use that one if there
+                                if (defaultConfiguredProject === undefined &&
+                                    index !== this.containingProjects.length - 1) {
+                                    defaultConfiguredProject = project.projectService.findDefaultConfiguredProject(this) || false;
+                                }
+                                if (defaultConfiguredProject === project) return project;
+                                if (!firstNonSourceOfProjectReferenceRedirect) firstNonSourceOfProjectReferenceRedirect = project;
+                            }
                             if (!firstConfiguredProject) firstConfiguredProject = project;
                         }
                         else if (project.projectKind === ProjectKind.External && !firstExternalProject) {
                             firstExternalProject = project;
                         }
                     }
-                    return firstConfiguredProject || firstExternalProject || this.containingProjects[0];
+                    return defaultConfiguredProject ||
+                        firstNonSourceOfProjectReferenceRedirect ||
+                        firstConfiguredProject ||
+                        firstExternalProject ||
+                        this.containingProjects[0];
             }
         }
 

src/testRunner/unittests/tsserver/configuredProjects.ts

@@ -846,7 +846,6 @@ namespace ts.projectSystem {
         });
 
         it("when multiple projects are open, detects correct default project", () => {
-            //const projectLocation: `/user/username/projects/myproject`;
             const barConfig: File = {
                 path: `${projectRoot}/bar/tsconfig.json`,
                 content: JSON.stringify({