There are proxies for OnStartRequest and OnStopRequest but in both cases the should have a lowercase O.

I am reading this right, aren't I? I think the objects being proxied are instances of CalDavMultigetSyncHandler etc., and therefore they have a lowercase O.

Hi Geoff, would fixing this bug have a positive impact on Bug 1642292 or it is not related?

It might. I'm not sure.

Pushed by geoff@darktrojan.net:
https://hg.mozilla.org/comm-central/rev/dc8975662a21
Fix LegacySAXResponse proxy trapping wrong targets. r=pmorris

Oh, and is there a bug to get rid of CalDavLegacySAXRequest?
https://searchfox.org/comm-central/rev/93db6b3c1403b18025e1ad81466979c5b4cd2772/calendar/providers/caldav/modules/CalDavRequest.jsm#476-479

(In reply to Geoff Lankow (:darktrojan) from comment #3)

It might. I'm not sure.

FYI, after testing with both https://ftp.mozilla.org/pub/thunderbird/nightly/2020/06/2020-06-23-10-34-34-comm-central/thunderbird-79.0a1.en-US.win64.zip (which does not include this patch) and https://ftp.mozilla.org/pub/thunderbird/nightly/2020/06/2020-06-24-10-56-34-comm-central/thunderbird-79.0a1.en-US.win64.zip (which include this patch)... I found no difference nor improvement with regards to Bug 1642292. Same issue occurs in both cases...

On a side note, the following crashes where encountered with both TB alpha above when trying to disable my CalDav calendar or cancel the authentication prompt that appears after enabling it... not directly related to this bug but maybe worth mentioning it fyi...
bp-10213894-1805-45a2-8718-8a1780200624
bp-e3c9b6e8-98f3-4bff-8b77-8c2990200624
bp-df076021-673f-4854-97d8-57c830200624
bp-e1f95118-5e2e-490f-8f26-51d630200624
bp-26b008d2-fe28-4abf-b205-3df640200624

(In reply to Richard Leger from comment #7)

(In reply to Geoff Lankow (:darktrojan) from comment #3)

It might. I'm not sure.

FYI, after testing with both https://ftp.mozilla.org/pub/thunderbird/nightly/2020/06/2020-06-23-10-34-34-comm-central/thunderbird-79.0a1.en-US.win64.zip (which does not include this patch) and https://ftp.mozilla.org/pub/thunderbird/nightly/2020/06/2020-06-24-10-56-34-comm-central/thunderbird-79.0a1.en-US.win64.zip (which include this patch)... I found no difference nor improvement with regards to Bug 1642292. Same issue occurs in both cases...

On a second side note, both TB alpha version above requires to right click couple of time on the "Syncrhonise Calendar" after enabling my CalDAV calendar (which cause a prompt for authentication) and authenticating... otherwise calendar is simply not loading... In previous version of TB such as 78.0b2 calendar start loading automatically and immediately after successful authentication... just for your information... not directly linked to this bug evidently...

Those crashes look like a variant of bug 1530177.

(In reply to Magnus Melin [:mkmelin] from comment #6)

Oh, and is there a bug to get rid of CalDavLegacySAXRequest?

I've just added bug 1648598 for that.

Thunderbird 78.0b4:
https://hg.mozilla.org/releases/comm-beta/rev/e2b6205a866d