add test for correct tab handling, docs

src/Buffer.test.ts

@@ -1312,6 +1312,13 @@ describe('Buffer', () => {
           terminal.buffer.lines.get(bufferIndex[0]).get(bufferIndex[1])[CHAR_DATA_CHAR_INDEX]);
       }
     });
+
+    it('should handle \t in lines correctly', () => {
+      const input = '\thttps://google.de';
+      terminal.writeSync(input);
+      const s = terminal.buffer.iterator(true).next().content;
+      assert.equal(s, Array(terminal.getOption('tabStopWidth') + 1).join(' ') + 'https://google.de');
+    });
   });
   describe('BufferStringIterator', function(): void {
     it('iterator does not overflow buffer limits', function(): void {

src/Buffer.ts

@@ -458,7 +458,9 @@ export class Buffer implements IBuffer {
       const length = (trimRight) ? line.getTrimmedLength() : line.length;
       for (let i = 0; i < length; ++i) {
         if (line.get(i)[CHAR_DATA_WIDTH_INDEX]) {
-          stringIndex -= line.get(i)[CHAR_DATA_CHAR_INDEX].length || 1; // WHITESPACE_CELL_CHAR.length
+          // empty cells report a string length of 0, but get replaced
+          // with a whitespace in translateToString, thus replace with 1
+          stringIndex -= line.get(i)[CHAR_DATA_CHAR_INDEX].length || 1;
         }
         if (stringIndex < 0) {
           return [lineIndex, i];