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Synthetic custom element lifecycle swallows DOM errors for `insertBefore`

1

### Description Because of the [global monkey-patching](https://github.com/salesforce/lwc/blob/7b0418c79e4159c5a8d0f8bda5ded0f1cf312ce7/packages/%40lwc/engine-dom/src/apis/create-element.ts#L63-L82) for synthetic custom element lifecycle, we actually swallow some errors that the browser would normally throw for improper usages of the `insertBefore` API....

nolanlawson

Use native `connectedCallback`/`disconnectedCallback` lifecycle hooks

2

We use non-standard mechanisms to trigger `connectedCallback`/`disconnectedCallback`. We should just use the native mechanism instead. https://github.com/salesforce/lwc/blob/551f6349421f3cfaf4e8c4a8d3d393b7549bb0ab/packages/@lwc/engine-dom/src/apis/create-element.ts#L50-L72 This has implications for backwards compat, because the timing and other subtleties will be...

nolanlawson

### Description The element returned by `createElement` is an HTML element with all of the `@api`-decorated properties from the provided component class. And we see, from usage, that it is...

wjhsf

List re-rendering causes DOM state to be lost

1

### Description When using a template list (`for:each` or `iterator:*`), if that list contains other LWC components, then after a reordering of the list, it is possible for a component...

nolanlawson

SSR hydration mismatch errors in `class` attributes

1

### Description I wanted to bring some subtle SSR hydration mismatch issue to your attention. This gist: If you happen to have a construct like `class={classGetter}` in a template, and...

seckardt

Support transforming `.mjs` files

2

**Is your feature request related to a problem? Please describe.** More and more libraries are leveraging the `.mjs` file extension for their ES Modules. The LWC transformer code only looks...

wes-goulet

### Description The current design of shadow DOM mixed mode is that `shadowSupportMode` only applies transitively to _children_, not to slot contents. However, similar to recent issues with native custom...

nolanlawson

_**Edit**: there is now an [RFC](https://github.com/salesforce/lwc-rfcs/pull/89)._ #4249 indicates that it may not be safe to compose native and synthetic lifecycle components together. For that reason, we should perhaps treat it...

nolanlawson

_**TL;DR**: No matter what, there will be some observable difference if a synthetic-lifecycle component is slotted into a native-lifecycle component._ As demonstrated by #4248, we have some issues where we...

nolanlawson