Delete specific Time Machine backup via GUI I would like to delete specific Time Machine backups, but not the oldest backup (which Time Machine will do automatically) but one in the middle. Simply dragging it (i.e., a folder such as for example /Backups.backupdb/computername/2018-06-20-041012) to the trash used to work, however, on recent systems it will result in one no longer being able to fully empty the Trash with the drive connected as you'll get the annoying "operation cannot be completed because Macintosh HD is in use" message, see here for discussion. 

So, how to properly delete middle backups? This appears to be the correct method via the command line. 

Yet, elsewhere (see answer by Glenn Leblanc) it is suggested that the correct method via user interface is by entering Time Machine, navigating to the Backups.backup/computername/ directory and using the context menu "Delete all backups of ..." to delete the backup.  Can anyone confirm if that is indeed the case?  

Because the contextual "Delete all backups of ..."  menu item is usually used in Time Machine to delete all backups of a particular item (such as a particular large file), which I understand as deleting that item from all backups (which can be useful to free up space). Now if this is true then "Delete all backups of 2018-06-20-041012", for example, would mean "delete all versions of all files backed up on that day" ... which would remove all backups of that machine (except for newly changed/added files after that date. 
Or is it that the same command behaves differently when applied to a particular file within a backup as opposed to a full backup folder? Could any of the Time Machine wizards here explain how this is supposed to be working?
To delete a specific backup enter Time Machine by selecting that option from the time machine icon in the menu bar. Once inside Time Machine, there will be a stack of windows. Click on the up arrow to the right of the window stack. This will start leafing thru the stack of backups with the dates shown in the top section of the windows. Run thru the stack until you get to the backup you want to remove. With the window for this backup at the front of the stack, click on the icon that looks like a gear located in the top part of the window below the date. One option that appears is to delete the backup. Choose that selection to delete that specific backup.