feat: disable deep merging theme configs by default

[curbengh]

What does it do?

Continuation of #3967. Requested by #3967 (comment) @Teekivi
deep merging theme.config and config.theme_config is considered less common use case.

cc #3964 @cmpute

Introduce a new option:

deepmerge_theme_configs: false

How to test

git clone -b optional-merge-theme https://github.com/curbengh/hexo.git
cd hexo
npm install
npm test

Screenshots

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

[SukkaW]

I don't think it is a common case.

theme_config is designed to merge with existed theme's _config.yml and only override existed value, which deep merge is doing exactly what it should do. And I believe the case @Teekivi provided is
in fact less common.

IMHO, the case @Teekivi provided is due to the bad-designed theme (which provides default items instead of giving example in _config.yml), not the fault of Hexo. After we bring up deep merge behavior, it seems that only hexo-theme-hermes is facing the issue?

---

Providing a case:

# theme's _config.yml
a:
  b:
    menu:
      d: 'e'
      f: 'g'
    sidebar:
      h: 'i'

# _config.yml
theme_config:
  a:
    b:
      menu:
        d: 'e'

Then they will be no sidebar config left. The @Teekivi cases only exists for theme's _config.yml looks like this:

# theme's _config.yml
# No less than 2 levels, which means override any 1st level won't affect other configurations 
a:
  b: e
  c: f
d: g

[Teekivi]

@SukkaW I'm quite new to the Hexo world, so you are probably right about my problem not being very common.

I understand that the "case" you provided is not meant as a good example, but it does show the limitations posed by default deep merge. As I said in #3967 (comment) deep merge makes it impossible to reorder or omit predefined dictionary/menu items solely via theme_config. So, in your example, you cannot change the order of d and f or omit one or both of them. That is - without changing theme source code.

I think that deep merge brings a lot more good than bad. But in my case, I experienced the less prevalent downside of it. So I think that as a possibility, we could expect future theme development to take the new behavior into account and not change it. Or we could make it globally opt-out or opt-in (the latter being this case), or figure out a good way to enable/disable deep merge selectively.

[curbengh]

IMHO, the case provided is due to the bad-designed theme (which provides default items instead of giving example in _config.yml)

👍

figure out a good way to enable/disable deep merge selectively

I could introduce an option, which is a reverse of this PR. But that option is more of a workaround for a theme which has invalid defaults, so the option wouldn't be useful.

[stevenjoezhang]

I have encountered this problem in the theme NexT: iissnan/hexo-theme-next#1861 theme-next/hexo-theme-next#878
The solution is not to preset any menu items - all default options should be commented. Maybe we need to remind all theme developers here to pay attention to this: https://github.com/hexojs/hexo-theme-unit-test

hexojs/hexo-theme-landscape#102

[FossPrime]

I have encountered this problem in the theme NexT: iissnan/hexo-theme-next#1861 theme-next/hexo-theme-next#878
The solution is not to preset any menu items - all default options should be commented. Maybe we need to remind all theme developers here to pay attention to this: https://github.com/hexojs/hexo-theme-unit-test

hexojs/hexo-theme-landscape#102

This is an issue on the landscape default theme... All the menu items are loaded by default.

I think there are a few compounding problems

• the schema should allow object values for widgets: [{ enabled: false, name: tag_cloud } ]
• we should be able to choose which config we wish to inherit from using Yaml. We can inherit the full config for demos, and the minimal one for serious work.