FLEX-35260 When calling localToGlobal() on gridColumnHeaderGroup, the result does not reflect its horizontal scroll position, which means that we need to include it manually (in GridHeaderViewLayout.getHeaderIndexAt()). However, the 'x' function argument is assumed to include it, so we're not adding it anymore when calling GridDimensionsView.getColumnIndexAt().
FLEX-35280FLEX-35260 Recent changes to FLEX-35267 made it so that the y value in GridHeaderViewLayout.getHeaderIndexAt() started to be considered, when before it was simply ignored. But one caller of this function, GridColumnHeaderGroup.eventToHeaderLocations(), simply used '0' for that argument, instead of the real value, knowing that (before these changes) it wasn't being considered. Now the real value is used.