Add e.preventDefault() to mouse event listeners

[vegetableman]

Platform: macOS Mojave Browser: Chrome 72.0.3626.96 Bug: Noticed inconsistency in mouseup being triggered. In Mac, I had to make sure to release the pressure from trackpad with intent in order for the event to be triggered. Solution: Adding e.preventDefault() to mouse event listeners fixes the issue. Reference: https://github.com/Microsoft/vscode/blob/2f3fc3f6bbf790763c72529390072528c95e9169/src/vs/base/browser/ui/sash/sash.ts#L274