Correct registerMarker docs

Fixes #4409

src/browser/Terminal.ts

@@ -917,7 +917,7 @@ export class Terminal extends CoreTerminal implements ITerminal {
     return this.buffer.markers;
   }
 
-  public addMarker(cursorYOffset: number): IMarker | undefined {
+  public addMarker(cursorYOffset: number): IMarker {
     return this.buffer.addMarker(this.buffer.ybase + this.buffer.y + cursorYOffset);
   }
 

src/browser/Types.d.ts

@@ -59,7 +59,7 @@ export interface IPublicTerminal extends IDisposable {
   registerLinkProvider(linkProvider: ILinkProvider): IDisposable;
   registerCharacterJoiner(handler: (text: string) => [number, number][]): number;
   deregisterCharacterJoiner(joinerId: number): void;
-  addMarker(cursorYOffset: number): IMarker | undefined;
+  addMarker(cursorYOffset: number): IMarker;
   registerDecoration(decorationOptions: IDecorationOptions): IDecoration | undefined;
   hasSelection(): boolean;
   getSelection(): string;

src/browser/public/Terminal.ts

@@ -156,7 +156,7 @@ export class Terminal implements ITerminalApi {
     this._checkProposedApi();
     this._core.deregisterCharacterJoiner(joinerId);
   }
-  public registerMarker(cursorYOffset: number = 0): IMarker | undefined {
+  public registerMarker(cursorYOffset: number = 0): IMarker {
     this._verifyIntegers(cursorYOffset);
     return this._core.addMarker(cursorYOffset);
   }

typings/xterm.d.ts

@@ -954,12 +954,11 @@ declare module 'xterm' {
     deregisterCharacterJoiner(joinerId: number): void;
 
     /**
-     * Adds a marker to the normal buffer and returns it. If the alt buffer is
-     * active, undefined is returned.
+     * Adds a marker to the normal buffer and returns it.
      * @param cursorYOffset The y position offset of the marker from the cursor.
      * @returns The new marker or undefined.
      */
-    registerMarker(cursorYOffset?: number): IMarker | undefined;
+    registerMarker(cursorYOffset?: number): IMarker;
 
     /**
      * (EXPERIMENTAL) Adds a decoration to the terminal using