When you open new windows of an application, like opening 4 windows of Google Chrome browser, they are shown separately not in a set as in Windows. In addition, when the taskbar gets crowded, it passes the borders of the screen. If you have small icons on your taskbar, they are no longer visible and the new windows are out of the screen. Simply there is no border on the taskbar.
Thanks
This is a known upstream issue that is highly unlikely to be resolved.
Upstream recommends using the icon tasklist applet.
Others use tint2 to display a panel with traditional task icons