Coalesce "All day events"

 * Cinnamon version: Cinnamon 5.2.7
   - Please specify if you are using the daily builds PPA
     (https://launchpad.net/~linuxmint-daily-build-team/+archive/ubuntu/daily-builds): No
 * Distribution: Linux Mint 20.3 Cinnamon

Preamble

I use the Calendar with public Holidays applet and filed an improvement issue against it: https://github.com/linuxmint/cinnamon-spices-applets/issues/4389

That applet's author @ccprog said:

While technically the event manager code is part of my applet, I am only merging that of the core Cinnamon calender applet. You should post your issue upstream. I am monitoring the code development there and will include improvements here.

so I repeat the issue here.

Since I do not use the core Cinnamon calendar applet, I cannot verify, if the core Cinnamon applet does show the same behaviour. So please feel free to close this issue, if it is not appropriate.

Issue

On this screenshot you can see, that each "All day" event, gets its own entry:

Expected behaviour

To save space (and to improve clarity!) it would be appreciated, if all "All day" events gets coalesced under one heading, e.g.:

Other information

Even better: All events starting at the same time should be coalesced under one heading, e.g. if there were a second event starting at 11:30 am, this should also be listed under the 11:30 am heading.

Start by locating the Cinnamon calendar applet's event-rendering entry point; the issue does not name a file or test. Reproduce the displayed event list, then verify that all-day events share one heading and that events with the same start time are grouped together without losing event details.