From @PerBothner's suggestion in #4519 (comment)

Right now we often either don't have docs linked to internal types (making development harder, especially for new contributors) or duplicate the docs which of course can easily suffer bitrot.

One solution is to reference xterm and then extend Pick<Terminal, 'apis'> like this:

If we do this we would want some rules around the usage, here are some ideas using Terminal.selectAll()/ISelectionService.selectAll() as an example.

1: Pull in any types referenced to a special types file

Example:

ApiTypes.ts

import { Terminal } from 'xterm';

// Each function used internally would have it's own type
export type IApiSelectAll = Pick<Terminal, 'selectAll'>;

Services.ts

export interface ISelectionService extends IApiSelectAll {
}

2: Pull in directly from the API

Services.ts

import { Terminal as IApiTerminal } from 'xterm';

export interface ISelectionService extends Pick<IApiTerminal, 'selectAll'> {
}

I think 1 is probably the nicest unless there are other ideas because:

• It's obvious across the codebase what IApi* types are
• It hides the type complexity inside the ApiTypes file
• Extending multiple is cleaner as a result
• No need to rename the Terminal as IApiTerminal import manually when a file hasn't referenced it yet

export interface ISelectionService extends IApiSelectAll,
                                           IApiSelectLines {
}

vs

export interface ISelectionService
  extends Pick<IApiTerminal, 'selectAll' | 'selectLines'> {
}