Try running pkmon and see what state PackageKit is in? I haven't seen this, could it be that it's just slowly downloading the updates? (i.e. pacman -Sy)

Created attachment 109936 [details]
screenshot

See the screenshot please.

I'm not calling you a liar, I need to understand the nature of the issue to be able to provide fixes.

Please provide the information I requested.

What information?
My screenshot shows pkmon result.
Updates download speed is fast, the bug described here occurs all the time, even when no update is found by "pacman -Syu" command.

You will have to open pkmon before opening discover:
$ pkmon
$ plasma-discover

Another thing to try is to call:
plasma-discover --backends packagekit-backend

To make sure it's PK that is failing and not others.

here is pkmon output

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daemon connected=1
network status=online
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 1      /2_dddcaccb
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/2_dddcaccb     allow_cancel 1
/2_dddcaccb     percentage   -1
/2_dddcaccb     role         get-repo-list
/2_dddcaccb     status       finished
Transactions:
 1      /3_adeebdba
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 1      /3_adeebdba
 2      /4_bdadabea
Transactions:
 1      /3_adeebdba
 2      /4_bdadabea
 3      /5_baecdbee
/3_adeebdba     allow_cancel 1
/3_adeebdba     percentage   -1
/3_adeebdba     role         resolve
/3_adeebdba     status       query
/4_bdadabea     allow_cancel 1
/4_bdadabea     percentage   -1
/4_bdadabea     role         resolve
/4_bdadabea     status       wait
/5_baecdbee     allow_cancel 1
/5_baecdbee     percentage   -1
/5_baecdbee     role         get-updates
/5_baecdbee     status       wait
/3_adeebdba     status       finished
Transactions:
 1      /4_bdadabea
 2      /5_baecdbee
/4_bdadabea     status       setup
/3_adeebdba     exit code: success
/4_bdadabea     status       query
/4_bdadabea     status       finished
Transactions:
 1      /5_baecdbee
/5_baecdbee     status       setup
/5_baecdbee     status       query
/5_baecdbee     status       download-packagelist
/4_bdadabea     exit code: success
/5_baecdbee     percentage   16
/5_baecdbee     percentage   33
/5_baecdbee     percentage   50
/5_baecdbee     percentage   66
/5_baecdbee     percentage   83
/5_baecdbee     percentage   84
/5_baecdbee     percentage   85
/5_baecdbee     percentage   86
/5_baecdbee     percentage   87
/5_baecdbee     percentage   88
/5_baecdbee     percentage   89
/5_baecdbee     percentage   90
/5_baecdbee     percentage   91
/5_baecdbee     percentage   92
/5_baecdbee     percentage   93
/5_baecdbee     percentage   94
/5_baecdbee     percentage   95
/5_baecdbee     percentage   96
/5_baecdbee     percentage   97
/5_baecdbee     percentage   98
/5_baecdbee     percentage   99
/5_baecdbee     percentage   100
/5_baecdbee     status       finished
/5_baecdbee     exit code: success
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plasma-discover --backends packagekit-backend
Using Wayland-EGL
Using the 'xdg-shell-v6' shell integration
setting currentApplicationBackend PackageKitBackend(0x55f8177d2510)
no packages for ""

When forcing only packagekit-backend, do you still have the issue?

discover started with
plasma-discover --backends packagekit-backend

is not affected by the issue described here.

I just noticed that discover is also affected on neon dev unstable if packagekit is not forced.

Interesting!

Could you try it with the other backends?
 * flatpak-backend
 * dummy-backend
 * kns-backend
 * snap-backend
 * packagekit-backend

I've really never seen this issue...

flatpak, dummy, packagekit -> enabled
kns -> disabled

my results

 * flatpak-backend - no problem
 * dummy-backend - discover shows no package
 * kns-backend - the problem described here occurs
 * snap-backend - discover shows no package
 * packagekit-backend - no problem

Git commit cb492acc1de9840e5f7bd55bdfd492355c6cb01c by Aleix Pol.
Committed on 29/01/2018 at 16:44.
Pushed by apol into branch 'Plasma/5.12'.

Don't let the fact that a backend is fetching block all backends

M  +2    -4    libdiscover/resources/ResourcesModel.cpp

https://commits.kde.org/discover/cb492acc1de9840e5f7bd55bdfd492355c6cb01c

problem described here is still happening on Arch Linux, discover 5.12.

(In reply to Dr. Chapatin from comment #14)
> problem described here is still happening on Arch Linux, discover 5.12.

Which problem? For me the "Check for updates" button is now enabled, but the sidebar still always shows "Checking for updates"

(In reply to Antonio Rojas from comment #15)
> (In reply to Dr. Chapatin from comment #14)
> > problem described here is still happening on Arch Linux, discover 5.12.
> 
> Which problem? For me the "Check for updates" button is now enabled, but the
> sidebar still always shows "Checking for updates"

Sorry, situation is a bit different now.
Sidebar still always shows "Checking for updates", "Check for updates" button is enabled but nothing happens when clicked.

*** Bug 390840 has been marked as a duplicate of this bug. ***

commit 83cde4f08fab5ac4c32f0bbeea8a306bbece1353
Author: Aleix Pol <aleixpol@kde.org>
Date:   Wed Feb 21 16:33:16 2018 +0100

    Also mark as not fetching after fetching updates
    
    Otherwise the UI keeps wanting to check updates
    
    BUG: 389072

Git commit 83cde4f08fab5ac4c32f0bbeea8a306bbece1353 by Aleix Pol.
Committed on 21/02/2018 at 15:33.
Pushed by apol into branch 'Plasma/5.12'.

Also mark as not fetching after fetching updates

Otherwise the UI keeps wanting to check updates

M  +1    -0    libdiscover/backends/KNSBackend/KNSBackend.cpp
M  +3    -0    libdiscover/backends/KNSBackend/tests/KNSBackendTest.cpp

https://commits.kde.org/discover/83cde4f08fab5ac4c32f0bbeea8a306bbece1353