fix(language-server): Do best-effort to initialize composite project when HTML is opened first

An upstream TypeScript performance-motivated change (
microsoft/TypeScript@02b07a1) made it so composite projects are not opened unless the opened file is in the includes/files list of the `tsconfig`. This means that external HTML files do not trigger project loading when they are the first and only file opened.

As a best-effort, we attempt to open a TS file with the same name, which
will most often be the file for the component of the external template.
This is not guaranteed to be the case, so the only way to guarantee this
works correctly would be for developers to add the `**/*.html` files to
the `includes` list of their `tsconfig[.lib].json`.

resolves #2149

Author: atscott

server/src/session.ts

@@ -821,7 +821,20 @@ export class Session {
       // The content could be newer than that on disk. This could be due to
       // buffer in the user's editor which has not been saved to disk.
       // See https://github.com/angular/vscode-ng-language-service/issues/632
-      const result = this.projectService.openClientFile(filePath, text, scriptKind);
+      let result = this.projectService.openClientFile(filePath, text, scriptKind);
+      // If the first opened file is an HTML file and the project is a composite/solution-style project with references,
+      // TypeScript will _not_ open a project unless the file is explicitly included in the files/includes list.
+      // This is quite unlikely to be the case for HTML files. As a best-effort to fix this, we attempt to open
+      // a TS file with the same name. Most of the time, this is going to be the component file for the external template.
+      // https://github.com/angular/vscode-ng-language-service/issues/2149
+      if (result.configFileName === undefined && languageId === LanguageId.HTML) {
+        const maybeComponentTsPath = filePath.replace(/\.html$/, '.ts');
+        if (!this.projectService.openFiles.has(this.projectService.toPath(maybeComponentTsPath))) {
+          this.projectService.openClientFile(maybeComponentTsPath);
+          this.projectService.closeClientFile(maybeComponentTsPath);
+          result = this.projectService.openClientFile(filePath, text, scriptKind);
+        }
+      }
 
       const {configFileName, configFileErrors} = result;
       if (configFileErrors && configFileErrors.length) {
@@ -831,24 +844,22 @@ export class Session {
       const project = configFileName ?
           this.projectService.findProject(configFileName) :
           this.projectService.getScriptInfo(filePath)?.containingProjects.find(isConfiguredProject);
-      if (!project) {
+      if (!project?.languageServiceEnabled) {
         return;
       }
-      if (project.languageServiceEnabled) {
-        // The act of opening a file can cause the text storage to switchToScriptVersionCache for
-        // version tracking, which results in an identity change for the source file. This isn't
-        // typically an issue but the identity can change during an update operation for template
-        // type-checking, when we _only_ expect the typecheck files to change. This _is_ an issue
-        // because the because template type-checking should not modify the identity of any other
-        // source files (other than the generated typecheck files). We need to ensure that the
-        // compiler is aware of this change that shouldn't have happened and recompiles the file
-        // because we store references to some string expressions (inline templates, style/template
-        // urls).
-        // Note: markAsDirty() is not a public API
-        (project as any).markAsDirty();
-        // Show initial diagnostics
-        this.requestDiagnosticsOnOpenOrChangeFile(filePath, `Opening ${filePath}`);
-      }
+      // The act of opening a file can cause the text storage to switchToScriptVersionCache for
+      // version tracking, which results in an identity change for the source file. This isn't
+      // typically an issue but the identity can change during an update operation for template
+      // type-checking, when we _only_ expect the typecheck files to change. This _is_ an issue
+      // because the because template type-checking should not modify the identity of any other
+      // source files (other than the generated typecheck files). We need to ensure that the
+      // compiler is aware of this change that shouldn't have happened and recompiles the file
+      // because we store references to some string expressions (inline templates, style/template
+      // urls).
+      // Note: markAsDirty() is not a public API
+      (project as any).markAsDirty();
+      // Show initial diagnostics
+      this.requestDiagnosticsOnOpenOrChangeFile(filePath, `Opening ${filePath}`);
     } catch (error) {
       if (this.isProjectLoading) {
         this.isProjectLoading = false;