Composition window doesn't keep IMAP connexion open

[Gordon Lack]

If you are writing a message and have only the Composition window open (not the main reading pane) then when this tries to save the current state of the reply in the Drafts folder (you've taken a short break whilst writing it) the save will fail.
The reason is that there is no connexion to the IMAP server (and the Drafts folder is on the IMAP server).
If you then start-up the main reading pane the save to the Drafts folder works.

ProblemType: Bug
DistroRelease: Ubuntu 20.10
Package: thunderbird 1:78.7.1+build1-0ubuntu0.20.10.4
ProcVersionSignature: Ubuntu 5.8.0-44.50-generic 5.8.18
Uname: Linux 5.8.0-44-generic x86_64
AddonCompatCheckDisabled: False
ApportVersion: 2.20.11-0ubuntu50.5
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: gml4410 2927 F.... pulseaudio
BuildID: 20210203182138
CasperMD5CheckResult: skip
Channel: Unavailable
CurrentDesktop: KDE
Date: Mon Mar 15 00:17:42 2021
Extensions: extensions.sqlite corrupt or missing
ForcedLayersAccel: False
IncompatibleExtensions: Unavailable (corrupt or non-existant compatibility.ini or extensions.sqlite)
InstallationDate: Installed on 2020-08-13 (213 days ago)
InstallationMedia: Kubuntu 20.04.1 LTS "Focal Fossa" - Release amd64 (20200731)
IpRoute:
 default via 192.168.1.1 dev wlp64s0 proto dhcp metric 600
 169.254.0.0/16 dev wlp64s0 scope link metric 1000
 192.168.1.0/24 dev wlp64s0 proto kernel scope link src 192.168.1.10 metric 600
Locales: extensions.sqlite corrupt or missing
PrefErrors: Unexpected character ',' before close parenthesis @ /usr/lib/thunderbird/omni.ja:greprefs.js:732
PrefSources: prefs.js
Profiles: Profile0 (Default) - LastVersion=78.7.1/20210203182138 (In use)
RunningIncompatibleAddons: False
SourcePackage: thunderbird
Themes: extensions.sqlite corrupt or missing
UpgradeStatus: Upgraded to groovy on 2020-10-25 (140 days ago)
dmi.bios.date: 11/15/2019
dmi.bios.release: 7.7
dmi.bios.vendor: INSYDE Corp.
dmi.bios.version: 1.07.07TPCS2
dmi.board.asset.tag: Tag 12345
dmi.board.name: N151CU
dmi.board.vendor: CLEVO
dmi.board.version: Not Applicable
dmi.chassis.asset.tag: No Asset Tag
dmi.chassis.type: 10
dmi.chassis.vendor: Notebook
dmi.chassis.version: N/A
dmi.ec.firmware.release: 7.7
dmi.modalias: dmi:bvnINSYDECorp.:bvr1.07.07TPCS2:bd11/15/2019:br7.7:efr7.7:svnPCSpecialistLTD:pnN150CU:pvrNotApplicable:rvnCLEVO:rnN151CU:rvrNotApplicable:cvnNotebook:ct10:cvrN/A:
dmi.product.family: Not Applicable
dmi.product.name: N150CU
dmi.product.sku: Not Applicable
dmi.product.version: Not Applicable
dmi.sys.vendor: PC Specialist LTD