A great Dropbox file exclusion trick no longer seems to work.
My Thunderbird profile is in Dropbox, mainly because I have years of historical email in the locally stored "Local Folders", so it is all synced to Dropbox in case of drive failure. But the file "parent.lock" that is opened when Thunderbird is running can't be synced by Dropbox, so just recently, probably because of some update, Dropbox is running high CPU repeatedly trying to sync the open file.
There used to be a great trick of creating a folder with the same name as a file you did not want to sync, and then excluding it from sync with the Dropbox "selective sync" feature, explained here, and then putting the file back, whereupon it would not be synced because it had the same name as the excluded folder.
However, I just tried it, and when you exclude the folder with selective sync, Dropbox now says "deselected files will only be available on Dropbox.com". So instead of removing the folder completely from sync and ignoring it, it puts it in cloud storage but remembers it. Then the moment you add the file back, with the same name as the folder, you get a conflict file created by Dropbox, and then it tries to sync the file again.
I tried deleting the cloud only folder on the Dropbox website, and trying the trick again, but that makes Dropbox forget about the sync exclusion, and the same thing happens all over again.
Can the trick in the link above still be done somehow? Somehow selectively exclude a folder from sync, but not put it in cloud storage, and just completely ignore that name?
EDIT: Ref my comment on the answer by @harrymc below, I could not figure out how to add the details to my comment, so add them here - the error I received:
PS C:\WINDOWS\system32> Set-Content -Path 'C:\Dropbox\System_Folder\Applications\Thunderbird\Profiles\profile.wms\parent.lock' -Stream com.dropbox.ignored -Value
Set-Content : Missing an argument for parameter 'Value'. Specify a parameter of type 'System.Object[]' and try again.
At line:1 char:132
+... \Profiles\profile.wms\parent.lock' -Stream com.dropbox.ignored -Value
~~~~~~
+ CategoryInfo : InvalidArgument: (:) [Set-Content], ParameterBindingException
+ FullyQualifiedErrorId : MissingArgument,Microsoft.PowerShell.Commands.SetContentCommand