fix(layout): correct mainContainerDimension

[prclin]

get mainContainerDimension(): dom.IDimension {
    return this._mainContainerDimension
  }

this always return the document.body dimension.
when container is not body, the quick input widget will offset;

so changes below

layout(): void {
  //this._mainContainerDimension = dom.getClientArea(window.document.body)
    this._mainContainerDimension = dom.getClientArea(this.mainContainer)

    this._onDidLayout.fire(this._mainContainerDimension)
  }

[CGNonofr]

Thanks!

[github-actions]

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