Why is there a flat seam in the bright strip of the quick window?

The bright seam in the express window is used to leave glue.

Extended data:

In architecture, a window refers to an opening built on a wall or roof to let light or air into a room. As early as the Sui Dynasty in China, there were windows that could be opened and closed automatically in the Guanwen Temple in the Wei Zi Palace.

During the Sui Dynasty in China, there were windows that could be opened and closed automatically in the Guan Wen Hall of Wei Zicheng in Luoyang Palace.

"There are 14 study rooms in front of Guanwen Temple, with window mattresses and kitchen curtains, which are extremely salty and beautiful. Every three rooms have square doors and brocade curtains, and there are two flying fairies on the ground. On the outdoor ground, the emperor is lucky in his study. Imperial secretary flew down with a censer, drew the curtain and went up. As soon as the emperor came out, he hung up and closed. " (History as a Mirror, Volume 182)

-There are fourteen luxurious libraries in front of Guanwen Temple, with brocade curtains hanging at the door, two flying immortals above the door, and organs outside. When Yang entered the library, the imperial secretary stepped forward and moved the organ. I saw the flying fairy falling, the brocade curtain rising, and the library windows and bookcase doors automatically opened. When Yang Guang leaves the library, everything will be restored automatically.