feat(tui): allow terminal and grid to disagree about character width

src/nvim/tui/tui.c

@@ -875,11 +875,56 @@ static void cursor_goto(UI *ui, int row, int col)
   ugrid_goto(grid, row, col);
 }
 
-static void clear_region(UI *ui, int top, int bot, int left, int right, int attr_id)
+static void print_spaces(UI *ui, int width)
+{
+  TUIData *data = ui->data;
+  UGrid *grid = &data->grid;
+
+  out(ui, data->space_buf, (size_t)width);
+  grid->col += width;
+  if (data->immediate_wrap_after_last_column) {
+    // Printing at the right margin immediately advances the cursor.
+    final_column_wrap(ui);
+  }
+}
+
+/// Move cursor to the position given by `row` and `col` and print the character in `cell`.
+///
+/// @param is_doublewidth  whether the character is double-width on the grid.
+///                        If true and the character is ambiguous-width, clear two cells.
+static void print_cell_at(UI *ui, int row, int col, UCell *cell, bool is_doublewidth)
 {
   TUIData *data = ui->data;
   UGrid *grid = &data->grid;
 
+  if (grid->row == -1 && cell->data[0] == NUL) {
+    // If cursor needs to repositioned and there is nothing to print, don't move cursor.
+    return;
+  }
+
+  cursor_goto(ui, row, col);
+
+  bool is_ambiwidth = utf_ambiguous_width(utf_ptr2char(cell->data));
+  if (is_ambiwidth && is_doublewidth) {
+    // Clear the two screen cells.
+    // If the character is single-width in the host terminal it won't change the second cell.
+    update_attrs(ui, cell->attr);
+    print_spaces(ui, 2);
+    cursor_goto(ui, row, col);
+  }
+
+  print_cell(ui, cell);
+
+  if (is_ambiwidth) {
+    // Force repositioning cursor after printing an ambiguous-width character.
+    grid->row = -1;
+  }
+}
+
+static void clear_region(UI *ui, int top, int bot, int left, int right, int attr_id)
+{
+  TUIData *data = ui->data;
+
   update_attrs(ui, attr_id);
 
   // Background is set to the default color and the right edge matches the
@@ -905,12 +950,7 @@ static void clear_region(UI *ui, int top, int bot, int left, int right, int attr
         UNIBI_SET_NUM_VAR(data->params[0], width);
         unibi_out(ui, unibi_erase_chars);
       } else {
-        out(ui, data->space_buf, (size_t)width);
-        grid->col += width;
-        if (data->immediate_wrap_after_last_column) {
-          // Printing at the right margin immediately advances the cursor.
-          final_column_wrap(ui);
-        }
+        print_spaces(ui, width);
       }
     }
   }
@@ -1302,8 +1342,8 @@ static void tui_flush(UI *ui)
       }
 
       UGRID_FOREACH_CELL(grid, row, r.left, clear_col, {
-        cursor_goto(ui, row, curcol);
-        print_cell(ui, cell);
+        print_cell_at(ui, row, curcol, cell,
+                      curcol < clear_col - 1 && (cell + 1)->data[0] == NUL);
       });
       if (clear_col < r.right) {
         clear_region(ui, row, row + 1, clear_col, r.right, clear_attr);
@@ -1439,8 +1479,8 @@ static void tui_raw_line(UI *ui, Integer g, Integer linerow, Integer startcol, I
     grid->cells[linerow][c].attr = attrs[c - startcol];
   }
   UGRID_FOREACH_CELL(grid, (int)linerow, (int)startcol, (int)endcol, {
-    cursor_goto(ui, (int)linerow, curcol);
-    print_cell(ui, cell);
+    print_cell_at(ui, (int)linerow, curcol, cell,
+                  curcol < endcol - 1 && (cell + 1)->data[0] == NUL);
   });
 
   if (clearcol > endcol) {
@@ -1458,8 +1498,8 @@ static void tui_raw_line(UI *ui, Integer g, Integer linerow, Integer startcol, I
     if (endcol != grid->width) {
       // Print the last char of the row, if we haven't already done so.
       int size = grid->cells[linerow][grid->width - 1].data[0] == NUL ? 2 : 1;
-      cursor_goto(ui, (int)linerow, grid->width - size);
-      print_cell(ui, &grid->cells[linerow][grid->width - size]);
+      print_cell_at(ui, (int)linerow, grid->width - size,
+                    &grid->cells[linerow][grid->width - size], size == 2);
     }
 
     // Wrap the cursor over to the next line. The next line will be