Bug 2403355

I cannot say if that issue occurs for 1 week or 1 month, just that it 90% was introduced some time in September or October 2025.

I usually snooze some (often more than 10 in parallel) appointments to the next 2 hours over the course of days or sometimes weeks, and process them when I have some time to spare. Some time ago, I opened my Thunderbird in the morning and no appointments had been there. I knew for sure several that should have been shown. However, I expected I just accidentally clicked "dismiss all" or so and no longer considered it (so don't know for sure the exact time). But I then saw that regularly appointments I snoozed disappear, usually not immediately when I snooze them the first time, but usually the next day. So in the evening, I snooze them once more, and in the morning they are gone. 

I thought first the persistent constant is the "day" or so, but then I tested more explicitly with two test appointments: I snoozed them around 5 times. Once in between, I closed Thunderbird and re-opened it immediately. It always reappeared at the expected time, also in the latter case. But once I kept Thunderbird closed at the very moment the reminder should have opened again, and opened Thunderbird 5 minutes after that instead, the reminders were lost and no longer appeared. I expect this is why the reminders of the night disappear, as the 2 hours I snooze will be over before Thunderbird is reopened the next morning.

However, when I tried to quickly reproduce this again, one appointment, snooze it the first time, close Thunderbird and wait until the reminder should be opened and open Thunderbird AFTER the reminder should have been opened, the reminder still was there at this one quick test. Today, I tested again: one appointment, snoozed 5 minutes, it reopened as it should, then I snoozed again 5 minutes and then I closed Thunderbird for 6 minutes (so one minute more), and reopened it: its lost again. At this final test, I ran Thunderbird through the terminal, but it does not contain any output except at some time the (in my opinion unrelated line) "Crash Annotation GraphicsCriticalError: |[0][GFX1-]: RenderCompositorSWGL failed mapping default framebuffer, no dt (t=99.8309) [GFX1-]: RenderCompositorSWGL failed mapping default framebuffer, no dt".

Another interesting phenomenon: I had a reminder of one appointment that got lost during the day (called for reminder 12 hours earlier) with the others. But at the very moment of the appointment, the reminder was opened again. To avoid false presumptions in the troubleshooting: I cannot exclude due to the nature of this very appointment that I dismissed it at the first reminder and so that the issue is that it was re-opened despite dismissal instead of snoozing with subsequent unintended disappearing and then reappearing at the time of the appointment (cannot say for sure which of the two). Most reminders I use are snoozed beyond the very time of the appointment. So I would not experience a reappearance at the very time of the appointment usually anyway given that that time is already passed when the disappearance takes place.

As mentioned I can say for sure 100% only at this time that when the machine shuts down in the evening and boots at the next morning, all reminders that I shift with 2 hours are gone, and I think I have not yet isolated / reproduced the exact condition that provokes the phenomenon. 

The issue seems to not impact if and when reminders are opened the first time: issues start with snoozing.

I consider this severe as people might not recognize the issue and miss appointments or forget about things. It is a matter of the user how severe consequences of this could be.

Unlikely to be related, but when I just closed the GUI-opened Thunderbird and tried to open my Thunderbird through a terminal to produce the final test with logs for you, the calendar mixed US with German calendar layout, and Sunday became the first day of the week rather than Monday. When I opened it again through the GUI-button, the layout was again a week beginning on Monday (I presume a locale issue, unlikely related, but mentioned for your consideration, just in case). As mentioned in the tests, both ways of opening Thunderbird seem affected anyway.

It does not correlate in time, but for some (about) months Thunderbird suffers from strong performance issues. From one moment to the next, I opened Thunderbird and it started to take several seconds to open the calendar and it regularly freezes several seconds in the beginning. Creating and opening appointments takes several seconds (it feels like up to half a minute) in the beginning, but once the Thunderbird calendar is open and one or two appointments had been opened, it works (my feeling is that it is always two calendar-related actions that freeze after opening Thunderbird, and then it works). I didn't report as I have not much time, but this one is unfortunately more severe.

The user journal logs of the current boot for `journalctl -r --boot=0 | grep thunderbird` (to avoid to miss anything that mentions thunderbird), which includes the final test, are (Due to the test I opened and closed Thunderbird more often than usual):

```
Oct 12 13:18:16 fedora thunderbird[24793]: Locale not supported by C library.
Oct 12 13:18:16 fedora thunderbird[24793]:         Using the fallback 'C' locale.: 'glib warning', file /builddir/build/BUILD/thunderbird-140.3.0-build/thunderbird-140.3.0/toolkit/xre/nsSigHandlers.cpp:201
Oct 12 13:18:16 fedora thunderbird[24793]: [24793, Main Thread] WARNING: Locale not supported by C library.
Oct 12 12:20:40 fedora thunderbird[7060]: Crash Annotation GraphicsCriticalError: |[0][GFX1-]: RenderCompositorSWGL failed mapping default framebuffer, no dt (t=120.183) [GFX1-]: RenderCompositorSWGL failed mapping default framebuffer, no dt
Oct 12 12:18:40 fedora thunderbird[7060]: Locale not supported by C library.
Oct 12 12:18:40 fedora thunderbird[7060]:         Using the fallback 'C' locale.: 'glib warning', file /builddir/build/BUILD/thunderbird-140.3.0-build/thunderbird-140.3.0/toolkit/xre/nsSigHandlers.cpp:201
Oct 12 12:18:40 fedora thunderbird[7060]: [7060, Main Thread] WARNING: Locale not supported by C library.
```

A comparison to a boot from April:

```
May 05 00:34:17 fedora thunderbird[18357]: Locale not supported by C library.
May 05 00:34:17 fedora thunderbird[18357]:         Using the fallback 'C' locale.: 'glib warning', file /builddir/build/BUILD/thunderbird-128.10.0-build/thunderbird-128.10.0/toolkit/xre/nsSigHandlers.cpp:187
May 05 00:34:17 fedora thunderbird[18357]: [18357, Main Thread] WARNING: Locale not supported by C library.
```

Let me know if you need more data or if I shall test something specific.

Reproducible: Sometimes

Steps to Reproduce:
Elaborated above, as far as possible. Reproducible not always / exact conditions not yet identified/isolated, though in some rough conditions it can be reproduced almost for certain (still reminders existing in the evening, snoozed for 2 hours, and thunderbird reopened the next morning many hours later).

As the title says, 1 "constant" that is always there, when the issue occurs, is that TB is closed during the exact time the snoozed reminder is to be opened again, but this "constant" on itself does not always reproduce the bug. At least in one test it did not (though I now consider the possibility that the Thunderbird is sooooo much slower than my clock that I accidentally opened Thunderbird before its own clock would have called the reminder of the test in which the condition did not lead to the bug: maybe the performance issues are here also involved, not sure).
Actual Results:
After some snoozes, reminders of appointments no longer appear, get lost and the user does not know about it

Expected Results:
Reminders of appointments disappear only after the user dismissed them

Additional Information:
My Fedora is up to date (F42 KDE), I update everything daily. I use only one calendar, which is a sqlite of Thunderbird in the backend. 

Current version:
Name            : thunderbird
Epoch           : 0
Version         : 140.3.0
Release         : 1.fc42
Architecture    : x86_64
Installed size  : 321.3 MiB
Source          : thunderbird-140.3.0-1.fc42.src.rpm
From repository : updates