left menu on backward 'Documentation' page behaves abnormally

[i11010520]

To reproduce:

1. open Documentation of jetzig.dev
2. click and jump to Downloads
3. go backward to Documentation with browser's back action

Then the hierarchy menu could not open anymore.
That's true on both Chrome and Safari.

[i11010520]

And, the rendering performance of jetzig.dev site seems not smooth.
That's not good for the demonstration of jetzig.