Preserve language picked for untitled text documents (fixes #16803)

src/vs/workbench/browser/parts/editor/editor.contribution.ts

@@ -96,6 +96,7 @@ Registry.as<IEditorRegistry>(EditorExtensions.Editors).registerEditor(
 
 interface ISerializedUntitledEditorInput {
 	resource: string;
+	modeId: string;
 }
 
 // Register Editor Input Factory
@@ -119,15 +120,15 @@ class UntitledEditorInputFactory implements IEditorInputFactory {
 			resource = URI.file(resource.fsPath); // untitled with associated file path use the file schema
 		}
 
-		const serialized: ISerializedUntitledEditorInput = { resource: resource.toString() };
+		const serialized: ISerializedUntitledEditorInput = { resource: resource.toString(), modeId: untitledEditorInput.getModeId() };
 
 		return JSON.stringify(serialized);
 	}
 
 	public deserialize(instantiationService: IInstantiationService, serializedEditorInput: string): EditorInput {
 		const deserialized: ISerializedUntitledEditorInput = JSON.parse(serializedEditorInput);
 
-		return this.untitledEditorService.createOrGet(URI.parse(deserialized.resource));
+		return this.untitledEditorService.createOrGet(URI.parse(deserialized.resource), deserialized.modeId);
 	}
 }
 

src/vs/workbench/common/editor/untitledEditorInput.ts

@@ -75,6 +75,14 @@ export class UntitledEditorInput extends AbstractUntitledEditorInput {
 		return this.resource;
 	}
 
+	public getModeId(): string {
+		if (this.cachedModel) {
+			return this.cachedModel.getModeId();
+		}
+
+		return this.modeId;
+	}
+
 	public getName(): string {
 		return this.hasAssociatedFilePath ? paths.basename(this.resource.fsPath) : this.resource.fsPath;
 	}