translateToString() adds one more entry to outColumns

Previously, for string 'ab好', outColumns=[0, 1, 2]. Now it becomes [0,
1, 2, 4]. This allows the user to know where does the last character
ends.

src/common/buffer/BufferLine.ts

@@ -510,10 +510,13 @@ export class BufferLine implements IBufferLine {
 
   /**
    * If outColumns is specified, it will be filled with column numbers such that
-   * returnedString[i] is display at outColumns[i] column. When a single cell is
-   * translated to multiple UTF-16 code units (e.g. surrogate pair) in the
-   * returned string, the corresponding entries in outColumns will have the same
-   * column number.
+   * returnedString[i] is display at outColumns[i] column.
+   * outColumns[returnedString.length] is where the character following
+   * returnedString will be displayed.
+   *
+   * When a single cell is translated to multiple UTF-16 code units (e.g.
+   * surrogate pair) in the returned string, the corresponding entries in
+   * outColumns will have the same column number.
    */
   public translateToString(trimRight?: boolean, startCol?: number, endCol?: number, outColumns?: number[]): string {
     startCol = startCol ?? 0;
@@ -537,6 +540,9 @@ export class BufferLine implements IBufferLine {
       }
       startCol += (content >> Content.WIDTH_SHIFT) || 1; // always advance by at least 1
     }
+    if (outColumns) {
+      outColumns.push(startCol);
+    }
     return result;
   }
 }