fix bug where tab width was determined based on screen column instead of line virtual column

src/gui/BufferPane.cc

@@ -35,7 +35,7 @@ void BufferPane::walk_line(const Buffer::Iterator & start_of_line, std::function
         if (code_point == '\t')
         {
             uint8_t tabstop = m_buffer->tabstop();
-            c_width = tabstop - screen_column % tabstop;
+            c_width = tabstop - virtual_column % tabstop;
         }
         else
         {