Allow collapsible sider to be used in fixed position

[SarhadSalam]

• I have searched the issues of this repository and believe that this is not a duplicate.

What problem does this feature solve?

Presently, in Layout Sider, the collapsible sider is in position:relative by default. To use a fixed sider, we must use a non-collapsible design because when position is set as fixed for the sider, the content of the Layout is manually set to 200px to the right.

Collapsible navigation is a great tool, but cannot be used in fixed mode and this causes multiple problem:

1. When we set 200px for margin-right, we have to manually change this value in the content css which is not good design. It's not good design because, let's say we have layout A, B, C. A is the parent component containing B and C. B is the sider, and C is the content. If we do not have redux setup (for simple projects), communication of this 200px/collapse status/animation of content sliding becomes a tedious task.

What does the proposed API look like?

The solution is really simple. For the first children inside a sider, there is a div with the class: .ant-layout-sider-children.

I was thinking, we check if(collapsible && position==fixed), then we add the following css rules to the above-mentioned class:

.ant-layout-sider-children {
     width: inherit;
     position: fixed;
}

This fixes all the problem, and we can use fixed collapsible navigation with a really easy fix.

I am willing to submit a PR to fix this, I just need to be guided in the right direction.

Thanks in advance!

[Enigama]

Is there any changes?