Once you start editing a block from the edit content panel (left side) in Modern Editor you might scroll away from that point in the preview pane (right side). Quickly find the block you were editing in the preview panel with the Location icon.
Location icon in Text block from the edit content panel (left side) of Modern Editor
Click on the Location icon to quickly find the block you are editing in your email. The preview panel (right side) of Modern Editor will highlight the block you are trying to locate and bring it to the top of the preview panel.