attempt to address regressions from #1684, #1745

This pull request eliminates the need to call L.DomUtil.getViewportOffset(). The test suites run successfully, and I do not see any regressions in the major browsers.

I did not remove L.DomUtil.getViewportOffset() and its tests in this pull request. That is a decision better made by others.

This attempts to address regressions from #1684 and #1745 .