User Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/78.0.3904.97 Safari/537.36

Steps to reproduce:

Move one or very few messages from inbox to a subfolder

Actual results:

EVERY SINGLE TIME I do so, this prompt shows up:

Do you wish to compact all local and offline folders to save disc space? This will save about 1,8 GB.

Note it says it will save about 1,8 GB (GIGABYTES)

I choose Yes, Thunderbirds compacts the folders, or it fakes it, it takes just a few seconds and says something like "Done compacting folders. 108 kB saved".

Note: it said it was gonna save almost 2 GB and now it says it saved one hundred and something kB, which isn't even nearly above the threshold for compacting folders in the first place.

Next time I open Thunderbird and do the same, the same happens.

Expected results:

If there is really 1,8 GB to be saved from compacting folders, then compact them effectively, save that much space, and stop asking.

If there's just a few kB to be saved, then don't ask to compact folders at all.

If for whatever reason you thought there were almost 2 GB to be saved and it turns out that it was only few kB, then learn from it or update/recompute whatever wrong information you had about folders, so you won't make the same miscalculation over and over again.

I use only pop3, I have recently massively deleted lots and lots of messages that may well have occupied more than one GB, plus some whole folder.
I also have some filters that delete messages when downloading new mail, not sure if that's relevant: (1) I would expect that to be implemented decently so that you don't actually store and immediately delete the messages causing fragmentation (or whatever the wasted space is called), but skip storing them altogether; (2) even if it does store and them delete them causing wasted space, that can't be so much that in a few hours I accumulate 1,8 G again. Plus it's always exactly 1,8 GB, that smells.

It's obvious that at some point something went wrong and some wrong information was stored and no matter how many times I compact folders that doesn't get corrected.

It's not really every single time.

I don't know exactly what triggers this, but it happens at least once a day.

We have a 4 year old unsolved version of this Bug 1260698 - Compact folders - shows wrong estimated and compacted size. Compacts too often.

It will be more interesting if you still see this when using version 68.