When I click on the download favicon button, it either says failed to download or download instantly, but in either case a progress bar window then shows up and get stuck at 0% or 1% and doesn't go away on its own and I have to click cancel. Happening for every website.
Please don't delete the issue template. Paste your debug info from the about dialog.
sorry about that.
KeePassXC - Version 2.3.4
Revision: 6fe821c
Libraries:
Operating system: Windows 10 (10.0)
CPU architecture: x86_64
Kernel: winnt 10.0.17134
Enabled extensions:
Doesn't matter if favicon is successfully downloaded or not, a progress bar window pops up as if the download is starting but just shows 0% and never goes away. Also sometimes when I click "download favicon", the progress bar shows but shows 0% and hangs, but then I click "cancel" the favicon is downloaded(???). Then after I click ok to close the edit window, the download favicon progress bar pops up again and hangs at 0%.
This happens for me also
KeePassXC - Version 2.3.4
Revision: 6fe821c
Libraries:
Operating system: OS X El Capitan (10.11)
CPU architecture: x86_64
Kernel: darwin 15.6.0
Enabled extensions:
Do you have that "google as fall-back" option checked? I think that is causing this issue. But if i uncheck the option I can't download any icon. Every website just returns the error "unable to fetch icon".
No, I don't have it checked, and the download works for me. I only get "unable to fetch icon" when the icon is not at /favicon.ico, I guess that is expected :)
When the "Google as fallback" option is checked, I even get three popups:

The popups appear for me in this order for namecheap.com, where the favicon download fails:
1) Download progress from namecheap.com
2) Download progress from namecheap.com
3) Download progress from google.com
4) All popups are closed, an error message is displayed
5) Download progress from namecheap.com
6) Download progress from namecheap.com
7) Download progress from google.com
Same issue on my platform.
KeePassXC - Version 2.3.4
Revision: 6fe821c
Libraries:
Operating system: Linux Mint 18.3
CPU architecture: x86_64
Kernel: linux 4.10.0-38-generic
Enabled extensions:
As @Olfrix points out, this isn't isolated to windows. I can confirm this is occurring on Linux as well. Sometimes I get multiple popup windows, other times I get a Download Progress popup that is stuck at 1%. I also have the Google as a fallback option checked, otherwise the favicons fail to download more >50% of the time.

KeePassXC - Version 2.3.4
Revision: 2cc52a8
Distribution: Snap
Libraries:
Operating system: Ubuntu Core 18
CPU architecture: x86_64
Kernel: linux 4.15.0-33-generic
Enabled extensions:
I thought I had fixed this when I also fixed the associated crash this caused. Unfortunately, looks like there is still a race condition that occurs. The dialogs should have an update timeout that autocloses them.
@droidmonkey it seems that setMinimumDuration() used in the fix for the crash only delays the time until the progress bar of the cancelled fetch appears. If I increase it to 4000, it appears after 4 seconds.
Most helpful comment
When the "Google as fallback" option is checked, I even get three popups:

The popups appear for me in this order for namecheap.com, where the favicon download fails:
1) Download progress from namecheap.com
2) Download progress from namecheap.com
3) Download progress from google.com
4) All popups are closed, an error message is displayed
5) Download progress from namecheap.com
6) Download progress from namecheap.com
7) Download progress from google.com