I reviewed the Hooks documentation. This suggests that you would generally interact with hooks (add, edit, delete) hooks through the fossil hook commands.

Then, I stumbled upon a "hooks" setting in <repo-uri>/setup_settings. The <repo-uri>/help?cmd=hooks page states:

The "hooks" setting contains JSON that describes all defined hooks. The value is > an array of objects. Each object describes a single hook. Example:

{
"type": "after-receive",  // type of hook
"cmd": "command-to-run",  // command to run
"seq": 50                 // run in this order
}

Although that setting is intended to contain JSON content, it is shown in the UI as a checkbox. If you modify the setting in the UI, it overwrites the JSON generated by the fossil hook commands.

Version information

$ fossil version
This is fossil version 2.16 [7aedd56758] 2021-07-02 12:46:01 UTC

Steps to reproduce

1. Start with no hooks:

   $ fossil hook list
   

2. Add a new hook:

   $ fossil hook add --command "go test ./..." --type before-commit
   $ fossil hook list
     0: type = before-commit
        command = go test ./...
        sequence = 10
   

3. Observe that the setting is JSON.

   $ fossil settings hooks
   hooks                (local)  [{"cmd":"go test ./...","type":"before-commit","seq":10}]
   

4. Navigate to the settings page in the UI, "check" the "hooks" setting and click "Apply Changes".

5. Review the hooks settings.

   $ fossil settings hooks
   hooks                (local)  1
   $ fossil hook list
     0: type =
        command =
        sequence = 0
   

Results

Modifying the hooks setting through the UI essentially breaks further use of the fossil hook commands. You cannot add/edit/delete any hooks. To get it back to a good state where you can use those commands, you must clear out the setting like so:

$ fossil settings hooks ''

I believe that we could solve this problem by showing the "hooks" setting as a text area in the UI.

Please let me know if there's any additional details I should provide.