Added --preserveWatchOutput flag

Description: "Whether to keep outdated console output in watch mode instead of clearing the screen."
Since the `pretty?` compiler options flag is marked as `@internal`, made this one too.

Author: Josh Goldberg

src/compiler/commandLineParser.ts

@@ -55,6 +55,13 @@ namespace ts {
             paramType: Diagnostics.FILE_OR_DIRECTORY,
             description: Diagnostics.Compile_the_project_given_the_path_to_its_configuration_file_or_to_a_folder_with_a_tsconfig_json,
         },
+        {
+            name: "preserveWatchOutput",
+            type: "boolean",
+            showInSimplifiedHelpView: false,
+            category: Diagnostics.Command_line_Options,
+            description: Diagnostics.Whether_to_keep_outdated_console_output_in_watch_mode_instead_of_clearing_the_screen,
+        },
         {
             name: "pretty",
             type: "boolean",

src/compiler/diagnosticMessages.json

@@ -3455,6 +3455,10 @@
         "category": "Message",
         "code": 6190
     },
+    "Whether to keep outdated console output in watch mode instead of clearing the screen.": {
+        "category": "Message",
+        "code": 6191
+    },
     "Variable '{0}' implicitly has an '{1}' type.": {
         "category": "Error",
         "code": 7005

src/compiler/types.ts

@@ -4014,6 +4014,7 @@ namespace ts {
         /*@internal*/ plugins?: PluginImport[];
         preserveConstEnums?: boolean;
         preserveSymlinks?: boolean;
+        /* @internal */ preserveWatchOutput?: boolean;
         project?: string;
         /* @internal */ pretty?: DiagnosticStyle;
         reactNamespace?: string;

src/compiler/watch.ts

@@ -513,7 +513,7 @@ namespace ts {
         }
 
         function clearHostScreen() {
-            if (watchingHost.system.clearScreen) {
+            if (watchingHost.system.clearScreen && !compilerOptions.preserveWatchOutput) {
                 watchingHost.system.clearScreen();
             }
         }

src/harness/unittests/tscWatchMode.ts

@@ -2100,6 +2100,21 @@ declare module "fs" {
             host.checkScreenClears(1);
         });
 
+        it("doesn't clear the console when it starts if --preserveWatchOutput is true", () => {
+            const file = {
+                path: "f.ts",
+                content: ""
+            };
+            const host = createWatchedSystem([file]);
+
+            createWatchModeWithoutConfigFile([file.path], host, {
+                preserveWatchOutput: true,
+            });
+            host.runQueuedTimeoutCallbacks();
+
+            host.checkScreenClears(0);
+        });
+
         it("clears the console on recompile", () => {
             const file = {
                 path: "f.ts",
@@ -2117,5 +2132,25 @@ declare module "fs" {
 
             host.checkScreenClears(2);
         });
+
+        it("doesn't clear the console on recompile if --preserveWatchOutput is true", () => {
+            const file = {
+                path: "f.ts",
+                content: ""
+            };
+            const host = createWatchedSystem([file]);
+            createWatchModeWithoutConfigFile([file.path], host, {
+                preserveWatchOutput: true,
+            });
+
+            const modifiedFile = {
+                ...file,
+                content: "//"
+            };
+            host.reloadFS([modifiedFile]);
+            host.runQueuedTimeoutCallbacks();
+
+            host.checkScreenClears(0);
+        });
     });
 }