** Specific: What do we want to achieve? **
The skinversion system in Vector adds unnecessary friction in the following ways:
- We have to add a `skinversion` field to any schema for any form of instrumentation to distinguish between the two experiences
- There is a risk of fatals in production. We have to exercise caution in code, as constructing a Skin is supposed to be cheap, and cannot access session information e.g. user preferences in certain parts of the flow.
- There is no way for extensions or user gadgets to ship different styles on the two Vectors. As a result, we are accumulating technical debt by shipping styles for two experiences together.
- Certain behaviours of skins are tied. As a result we've had no choice but to make changes that impact legacy and modern Vector (for example wrapping menu items in span in legacy Vector)
There are some benefits of the existing system that we want to keep:
* Ability for user gadgets/styles/scripts to load in both experiences.
** Measurable: How will we know when we've reached our goal? **
There should be two skins in the user preferences for Vector. These should behave like any other skin e.g. MonoBook / Timeless
** Achievable: What support will we need to achieve our goal? **
The performance team have raised various concerns with the approach so are blockers for completing this work.
** Relevant: Is this goal worthwhile? **
This goal should certainly be completed before the end of the project. The skin version code is very domain-specific, and will likely cause confusion if we were to switch projects and come back to it, say a year later.
Separating the skins also allows us to retire/hide the legacy skin in the long-term future (although I don't think we've ever do this in practice), as well as allowing other teams to maintain ownership. More importantly it allows us to work on modern Vector without having concern for impacting the legacy experience which it is tied to.
** Time-bound: What is the time frame? Can we achieve this goal in the timeframe I've set? **
# More information
One of the goals of the desktop improvements project was to minimize disruption to end-users gadgets and site scripts and ensure that the new Vector is compatible with all existing gadgets. For this reason, we should split the skins into 2 different keys with shared wiki pages/gadgets
# Phase 1 - Skin separation - Acceptance criteria
 Vector will be broken into [[ https://gerrit.wikimedia.org/r/c/mediawiki/core/+/721106 | two internal skins ]] that are hidden in user preferences. These will have skin keys vector19 and vector21
 Vector will be the default skin. The Vector constructor, will make use of SkinFactory::getSkinOptions to work out which skins to apply
 The skin key for Vector will be "vector"
 No change to end user experience.
# Phase 2 - Prepare and QA the skins - Acceptance criteria
 All skinStyles that declare vector, will now declare vector19 and vector22. Currently this looks like it will be a manual job (see T288857)
 We will undergo QA on both skins: ?useskin=vector19 and ?useskin=vector22 and ensure they are compatible with common gadgets and scripts.
 Both of the new skins should load MediaWiki:Vector.css and MediaWiki:Vector.js. This is possible by using ResourceLoaderSiteModulePages and ResourceLoaderSiteStylesModulePages hooks.
 A user script/stylesheet e.g User:Jdlrobson/Vector.css should apply to both skins. A new hook is needed to support this use case.
 Review the skin preference instrumentation with a QA engineer. Verify it gives us information we need and document any changes.
 Review the skin preference version workflow with designer.
# Phase 3 - Split the skins - Acceptance criteria
 The skins vector19 and vector21 will be made internal inside configuration using wgSkipSkins
 The internal flag will be dropped inside Vector skin.json
 Once the train has rolled out we will update the configuration, changing the default skin to "vector19" or "vector21" on desktop improvements projects. `vector` will be added to `wgSkipSkins` and `vector19` and `vector21` removed.
 Provided everything has gone to plan, the "vector" key inside Vector's skin registration will be removed and removed from wgSkipSkins