Clickable area for spit view divider is very narrow

[craxal]

The split view divider only has a 1px wide clickable area, making it difficult for users to "grab". The divider between the tree view and the explorers has a better width (about 5px), which we should also use for the split view.

[richardcho-msft]

@JasonYeMSFT I think you assigned to wrong person :)

[JasonYeMSFT]

@richardcho-msft Sorry, I fixed it.

[richardMSFT]

Implemented and fix should be shipped with 1.27.0