code: function _mergePanes() {
          if (_panes.hasOwnProperty(SECOND_PANE)) {
  
              var firstPane = _panes[FIRST_PANE],
                  secondPane = _panes[SECOND_PANE],
                  fileList = secondPane.getViewList(),
                  lastViewed = getCurrentlyViewedFile();
  
              Resizer.removeSizable(firstPane.$el);
              firstPane.mergeFrom(secondPane);
  
              exports.trigger("workingSetRemoveList", fileList, secondPane.id);
  
              setActivePaneId(firstPane.id);
  
              secondPane.$el.off(".mainview");
              secondPane.off(".mainview");
  
              secondPane.destroy();
              delete _panes[SECOND_PANE];
              exports.trigger("paneDestroy", secondPane.id);
              exports.trigger("workingSetAddList", fileList, firstPane.id);
  
              _mruList.forEach(function (record) {
                  if (record.paneId === secondPane.id) {
                      record.paneId = firstPane.id;
                  }
              });
  
              _$el.removeClass("split-" + _orientation.toLowerCase());
              _orientation = null;
              // this will set the remaining pane to 100%
              _initialLayout();
  
              exports.trigger("paneLayoutChange", _orientation);
  
              // if the current view before the merger was in the pane
              //  that went away then reopen it so that it's now the current view again
              if (lastViewed && getCurrentlyViewedFile() !== lastViewed) {
                  exports._open(firstPane.id, lastViewed);
              }
          }
      }
docs: Merges second pane into first pane and opens the current file
  @private