neovim/runtime/doc/develop.txt

*develop.txt*          Nvim


                            NVIM REFERENCE MANUAL


Development of Nvim                                     *development* *dev*

This reference describes design constraints and guidelines, for developing
Nvim applications or Nvim itself. See |dev-arch| for discussion of Nvim's
architecture and internal concepts.

Nvim is free and open source.  Everybody is encouraged to contribute.
    https://github.com/neovim/neovim/blob/master/CONTRIBUTING.md

                                      Type |gO| to see the table of contents.

==============================================================================
Design goals                                            *design-goals*

Most important things come first (roughly).  Some items conflict; this is
intentional.  A balance must be found.


NVIM IS... IMPROVED                                     *design-improved*

The Neo bits of Nvim should make it a better Vim, without becoming a
completely different editor.
- In matters of taste, prefer Vim/Unix tradition. If there is no relevant
  Vim/Unix tradition, consider the "common case".
- There is no limit to the features that can be added.  Select new features
  based on (1) what users ask for, (2) how much effort it takes to implement
  and (3) someone actually implementing it.
- Backwards compatibility is a feature.  The RPC API in particular should
  never break.


NVIM IS... WELL DOCUMENTED                              *design-documented*

- A feature that isn't documented is a useless feature.  A patch for a new
  feature must include the documentation.
- Documentation should be comprehensive and understandable.  Use examples.
- Don't make the text unnecessarily long.  Less documentation means that an
  item is easier to find.


NVIM IS... FAST AND SMALL                               *design-speed-size*

Keep Nvim small and fast. This directly affects versatility and usability.
- Computers are becoming faster and bigger each year.  Vim can grow too, but
  no faster than computers are growing.  Keep Vim usable on older systems.
- Many users start Vim from a shell very often.  Startup time must be short.
- Commands must work efficiently.  The time they consume must be as small as
  possible.  Useful commands may take longer.
- Don't forget that some people use Vim over a slow connection.  Minimize the
  communication overhead.
- Vim is a component among other components.  Don't turn it into a massive
  application, but have it work well together with other programs
  ("composability").


NVIM IS... MAINTAINABLE                                 *design-maintain*

- The source code should not become a mess.  It should be reliable code.
- Use comments in a useful way!  Quoting the function name and argument names
  is NOT useful.  Do explain what they are for.
- Porting to another platform should be made easy, without having to change
  too much platform-independent code.
- Use the object-oriented spirit: Put data and code together.  Minimize the
  knowledge spread to other parts of the code.


NVIM IS... NOT                                          *design-not*

Nvim is not an operating system; instead it should be composed with other
tools or hosted as a component. Marvim once said: "Unlike Emacs, Nvim does not
include the kitchen sink... but it's good for plumbing."


==============================================================================
Developer guidelines                                    *dev-guidelines*


PROVIDERS                                               *dev-provider*

A primary goal of Nvim is to allow extension of the editor without special
knowledge in the core.  Some core functions are delegated to "providers"
implemented as external scripts.

Examples:

1. In the Vim source code, clipboard logic accounts for more than 1k lines of
   C source code (ui.c), to perform two tasks that are now accomplished with
   shell commands such as xclip or pbcopy/pbpaste.

2. Python scripting support: Vim has three files dedicated to embedding the
   Python interpreter: if_python.c, if_python3.c and if_py_both.h. Together
   these files sum about 9.5k lines of C source code. In contrast, Nvim Python
   scripting is performed by an external host process implemented in ~2k lines
   of Python.

The provider framework invokes Vimscript from C.  It is composed of two
functions in eval.c:

- eval_call_provider({name}, {method}, {arguments}, {discard}): Calls
  `provider#{name}#Call` with {method} and {arguments}. If {discard} is true, any
  value returned by the provider will be discarded and empty value will be
  returned.
- eval_has_provider({name}): Checks the `g:loaded_{name}_provider` variable
  which must be set to 2 by the provider script to indicate that it is
  "enabled and working". Called by |has()| to check if features are available.

For example, the Python provider is implemented by the
"autoload/provider/python.vim" script, which sets `g:loaded_python_provider`
to 2 only if a valid external Python host is found.  Then `has("python")`
reflects whether Python support is working.

                                                        *provider-reload*
Sometimes a GUI or other application may want to force a provider to
"reload".  To reload a provider, undefine its "loaded" flag, then use
|:runtime| to reload it: >vim

    :unlet g:loaded_clipboard_provider
    :runtime autoload/provider/clipboard.vim


DOCUMENTATION                                           *dev-doc*

- "Just say it". Avoid mushy, colloquial phrasing in all documentation
  (docstrings, user manual, website materials, newsletters, …). Don't mince
  words. Personality and flavor, used sparingly, are welcome--but in general,
  optimize for the reader's time and energy: be "precise yet concise".
    - Prefer the active voice: "Foo does X", not "X is done by Foo".
    - "The words you choose are an essential part of the user experience."
      https://developer.apple.com/design/human-interface-guidelines/writing
    - "...without being overly colloquial or frivolous."
      https://developers.google.com/style/tone
- Write docstrings (as opposed to inline comments) with present tense ("Gets"),
  not imperative ("Get"). This tends to reduce ambiguity and improve clarity
  by describing "What" instead of "How". >
    ✅ OK:
    /// Gets a highlight definition.
    ❌ NO:
    /// Get a highlight definition.
- Avoid starting docstrings with "The" or "A" unless needed to avoid
  ambiguity. This is a visual aid and reduces noise. >
    ✅ OK:
    /// @param dirname Path fragment before `pend`
    ❌ NO:
    /// @param dirname The path fragment before `pend`
- Vim differences:
    - Do not prefix help tags with "nvim-". Use |vim_diff.txt| to catalog
      differences from Vim; no other distinction is necessary.
    - If a Vim feature is removed, delete its help section and move its tag to
      |vim_diff.txt|.
- Mention deprecated features in |deprecated.txt| and delete their old doc.
- Use consistent language.
    - "terminal" in a help tag always means "the embedded terminal emulator",
      not "the user host terminal".
    - Use "tui-" to prefix help tags related to the host terminal, and "TUI"
      in prose if possible.
- Rough guidelines on where Lua documentation should end up:
    - Nvim API functions `vim.api.nvim_*` should be in `api.txt`.
    - If the module is big and not relevant to generic and lower-level Lua
      functionality, then it's a strong candidate for separation. Example:
      `treesitter.txt`
    - Otherwise, add them to `lua.txt`

Documentation format ~

For Nvim-owned docs, use the following strict subset of "vimdoc" to ensure
the help doc renders nicely in other formats (such as HTML:
https://neovim.io/doc/user ).

Strict "vimdoc" subset:

- Use lists (like this!) prefixed with "-" or "•", for adjacent lines that you
  don't want to auto-wrap. Lists are always rendered with "flow" layout
  (soft-wrapped) instead of preformatted (hard-wrapped) layout common in
  legacy :help docs.
  - Limitation: currently the parser https://github.com/neovim/tree-sitter-vimdoc
    does not understand numbered listitems, so use a bullet symbol (- or •)
    before numbered items, e.g. "• 1." instead of "1.".
- Separate blocks (paragraphs) of content by a blank line.
- Do not use indentation in random places—that prevents the page from using
  "flow" layout. If you need a preformatted section, put it in
  a |help-codeblock| starting with ">".
- Parameters and fields are documented as `{foo}`.
- Optional parameters and fields are documented as `{foo}?`.

C docstrings ~

Nvim API documentation lives in the source code, as docstrings (doc
comments) on the function definitions.  The |api| :help is generated
from the docstrings defined in src/nvim/api/*.c.

Docstring format:
- Lines start with `///`
- Special tokens start with `@` followed by the token name:
  `@note`, `@param`, `@return`
- Markdown is supported.
- Tags are written as `[tag]()`.
- References are written as `[tag]`
- Use ``` for code samples.
  Code samples can be annotated as `vim` or `lua`

Example: the help for |nvim_open_win()| is generated from a docstring defined
in src/nvim/api/win_config.c like this: >

    /// Opens a new window.
    /// ...
    ///
    /// Example (Lua): window-relative float
    ///
    /// ```lua
    /// vim.api.nvim_open_win(0, false, {
    ///   relative='win',
    ///   row=3,
    ///   col=3,
    ///   width=12,
    ///   height=3,
    /// })
    /// ```
    ///
    /// @param buffer Buffer to display
    /// @param enter  Enter the window
    /// @param config Map defining the window configuration. Keys:
    ///   - relative: Sets the window layout, relative to:
    ///      - "editor" The global editor grid.
    ///      - "win"    Window given by the `win` field.
    ///      - "cursor" Cursor position in current window.
    /// ...
    /// @param[out] err Error details, if any
    ///
    /// @return Window handle, or 0 on error


Lua docstrings ~
                                                        *dev-lua-doc*
Lua documentation lives in the source code, as docstrings on the function
definitions.  The |lua-vim| :help is generated from the docstrings.

Docstring format:
- Use LuaCATS annotations: https://luals.github.io/wiki/annotations/
- Markdown is supported.
- Tags are written as `[tag]()`.
- References are written as `[tag]`
- Use ``` for code samples.
  Code samples can be annotated as `vim` or `lua`
- Use `@since <api-level>` to note the |api-level| when the function became
  "stable". If `<api-level>` is greater than the current stable release (or
  0), it is marked as "experimental".
  - See scripts/util.lua for the mapping of api-level to Nvim version.
- Use `@nodoc` to prevent documentation generation.
- Use `@inlinedoc` to inline `@class` blocks into `@param` blocks.
  E.g. >lua
      --- Object with fields:
      --- @class myOpts
      --- @inlinedoc
      ---
      --- Documentation for some field
      --- @field somefield? integer

      --- @param opts? myOpts
      function foo(opts)
      end
<

  Will be rendered as: >vimdoc

      foo({opts})

          Parameters:
            - {opts}? (table) Object with the fields:
                      - {somefield}? (integer) Documentation
                        for some field
<
- Files declared as `@meta` are only used for typing and documentation (similar to "*.d.ts" typescript files).

Example: the help for |vim.paste()| is generated from a docstring decorating
vim.paste in runtime/lua/vim/_editor.lua like this: >

    --- Paste handler, invoked by |nvim_paste()| when a conforming UI
    --- (such as the |TUI|) pastes text into the editor.
    ---
    --- Example: To remove ANSI color codes when pasting:
    ---
    --- ```lua
    --- vim.paste = (function()
    ---   local overridden = vim.paste
    ---   ...
    --- end)()
    --- ```
    ---
    --- @since 12
    --- @see |paste|
    ---
    --- @param lines  ...
    --- @param phase  ...
    --- @returns false if client should cancel the paste.


LUA STDLIB DESIGN GUIDELINES                            *dev-lua*

See also |dev-naming|.

- Keep the core Lua modules |lua-stdlib| simple. Avoid elaborate OOP or
  pseudo-OOP designs. Plugin authors just want functions to call, not a big,
  fancy inheritance hierarchy.
- Avoid requiring or returning special objects in the Nvim stdlib. Plain
  tables or values are easier to serialize, easier to construct from literals,
  easier to inspect and print, and inherently compatible with all Lua plugins.
  (This guideline doesn't apply to opaque, non-data objects like `vim.cmd`.)
- stdlib functions should follow these common patterns:
  - Return |lua-result-or-message| (`any|nil,nil|string`) to communicate
    failure, or choose from |dev-error-patterns| when appropriate.
  - Accept iterable instead of only table.
    - Note: in some cases iterable doesn't make sense, e.g. spair() sorts the
      input by definition, so there is no reason for it to accept an iterable,
      because the input needs to be "reified"; it can't operate on a "stream".
  - Return an iterable (generator) instead of table, if possible.
  - Mimic the pairs() or ipairs() interface if the function is intended for
    use in a |for-in| loop.

                                                          *dev-error-patterns*
To communicate failure to a consumer, choose from these patterns (in order of
preference):
1. `retval, errmsg`
  - When failure is normal, or when it is practical for the consumer to
    continue (fallback) in some other way. See |lua-result-or-message|.
2. optional result, no errormsg
  - Special case of 1. When there is only a single case of "doesn't exist"
    (e.g. cache lookup, dict lookup).
3. `error("no luck")`
  - For invalid state ("must not happen"), when failure is exceptional, or at
    a low level where the consumers are unlikely to handle it in a meaningful
    way. Advantage is that propagation happens for free and it's harder to
    accidentally swallow errors. (E.g. using
    `uv_handle/pipe:write()` without checking return values is common.)
4. `on_error` parameter
  - For async and "visitors" traversing a graph, where many errors may be
    collected while work continues.
5. `vim.notify` (sometimes with optional `opts.silent` (async, visitors ^))
  - High-level / application-level messages. End-user invokes these directly.

                                                        *dev-patterns*
Interface conventions ~

Where possible, these patterns apply to _both_ Lua and the API:

- When accepting a buffer id, etc., 0 means "current buffer", nil means "all
  buffers".  Likewise for window id, tabpage id, etc.
  - Examples: |vim.lsp.codelens.clear()| |vim.diagnostic.enable()|
- Any function signature that accepts a callback (example: |table.foreach()|)
  should place it as the LAST parameter (after opts), if possible (or ALWAYS
  for "continuation callbacks"—functions called exactly once).
  - Improves readability by placing the less "noisy" arguments near the start.
  - Consistent with luv.
  - Useful for future async lib which transforms functions of the form
    `function(<args>, cb(<ret)>))` => `function(<args>) -> <ret>`.
  - Example: >lua
    -- ✅ OK:
    filter(…, opts, function() … end)
    -- ❌ NO:
    filter(function() … end, …, opts)
    -- ❌ NO:
    filter(…, function() … end, opts)
- "Enable" ("toggle") interface and behavior:
  - `enable(…, nil)` and `enable(…, {buf=nil})` are synonyms and control the
    the "global" enablement of a feature.
    - `is_enabled(nil)` and `is_enabled({buf=nil})`, likewise, query the
      global state of the feature.
  - `enable(…, {buf: number})` sets a buffer-local "enable" flag.
    - `is_enabled({buf: number})`, likewise, queries the buffer-local state of
      the feature.
  - See |vim.lsp.inlay_hint.enable()| and |vim.lsp.inlay_hint.is_enabled()|
    for a reference implementation of these "best practices".
  - NOTE: open questions: https://github.com/neovim/neovim/issues/28603

API DESIGN GUIDELINES                                   *dev-api*

See also |dev-naming|.

- When adding an API, check the following:
  - What precedents did you draw from? How does your solution compare to them?
  - Does your new API allow future expansion? How? Or why not?
  - Is the new API similar to existing APIs? Do we need to deprecate the old ones?
  - Did you cross-reference related concepts in the docs?
- Avoid "mutually exclusive" parameters--via constraints or limitations, if
  necessary. For example nvim_create_autocmd() has mutually exclusive
  "callback" and "command" args; but the "command" arg could be eliminated by
  simply not supporting Vimscript function names, and treating a string
  "callback" arg as an Ex command (which can call Vimscript functions). The
  "buffer" arg could also be eliminated by treating a number "pattern" as
  a buffer number.
- Avoid functions that depend on cursor position, current buffer, etc. Instead
  the function should take a position parameter, buffer parameter, etc.

Where things go ~

- API (libnvim/RPC): exposes low-level internals, or fundamental things (such
  as `nvim_exec_lua()`) needed by clients or C consumers.
- Lua stdlib = high-level functionality that builds on top of the API.

NAMING GUIDELINES                                       *dev-naming*

Naming is exceedingly important: the name of a thing is the primary interface
for uses it, discusses it, searches for it, shares it... Consistent
naming in the stdlib, API, and UI helps both users and developers discover and
intuitively understand related concepts ("families"), and reduces cognitive
burden. Discoverability encourages code re-use and likewise avoids redundant,
overlapping mechanisms, which reduces code surface-area, and thereby minimizes
bugs...

Naming conventions ~

In general, look for precedent when choosing a name, that is, look at existing
(non-deprecated) functions. In particular, see below...

                                                        *dev-name-common*
Use existing common {verb} names (actions) if possible:
    - add:          Appends or inserts into a collection
    - attach:       Listens to something to get events from it (TODO: rename to "on"?)
    - call:         Calls a function
    - cancel:       Cancels or dismisses an event or interaction, typically
                    user-initiated and without error. (Compare "abort", which
                    cancels and signals error/failure.)
    - clear:        Clears state but does not destroy the container
    - create:       Creates a new (non-trivial) thing (TODO: rename to "def"?)
    - del:          Deletes a thing (or group of things)
    - detach:       Dispose attached listener (TODO: rename to "un"?)
    - enable:       Enables/disables functionality. Signature should be
                    `enable(enable?:boolean, filter?:table)`.
    - eval:         Evaluates an expression
    - exec:         Executes code, may return a result
    - fmt:          Formats
    - get:          Gets things (often by a query)
    - inspect:      Presents a high-level, often interactive, view
    - is_enabled:   Checks if functionality is enabled.
    - open:         Opens something (a buffer, window, …)
    - parse:        Parses something into a structured form
    - set:          Sets a thing (or group of things)
    - start:        Spin up a long-lived process. Prefer "enable" except when
                    "start" is obviously more appropriate.
    - stop:         Inverse of "start". Teardown a long-lived process.
    - try_{verb}:   Best-effort operation, failure returns null or error obj

Do NOT use these deprecated verbs:
    - disable:      Prefer `enable(enable: boolean)`.
    - exit:         Prefer "cancel" (or "stop" if appropriate).
    - is_disabled:  Prefer `is_enabled()`.
    - list:         Redundant with "get"
    - notify:       Redundant with "print", "echo"
    - show:         Redundant with "print", "echo"
    - toggle:       Prefer `enable(not is_enabled())`.

Use consistent names for {topic} in API functions: buffer is called "buf"
everywhere, not "buffer" in some places and "buf" in others.
    - buf:          Buffer
    - chan:         |channel|
    - cmd:          Command
    - cmdline:      Command-line UI or input
    - fn:           Function
    - hl:           Highlight
    - pos:          Position
    - proc:         System process
    - tabpage:      Tabpage
    - win:          Window

Do NOT use these deprecated nouns:
    - buffer        Use "buf" instead
    - callback      Use on_foo instead
    - command       Use "cmd" instead
    - window        Use "win" instead

                                                        *dev-name-events*
Use the "on_" prefix to name event-handling callbacks and also the interface for
"registering" such handlers (on_key). The dual nature is acceptable to avoid
a confused collection of naming conventions for these related concepts.

Editor |events| (autocommands) are historically named like: >
    {Noun}{Event}

Use this format to name API (RPC) events: >
    nvim_{noun}_{event-name}_event

Example: >
    nvim_buf_changedtick_event
<
                                                        *dev-api-name*
Use this format to name new RPC |API| functions: >
    nvim_{topic}_{verb}_{arbitrary-qualifiers}

Do not add new nvim_buf/nvim_win/nvim_tabpage APIs, unless you are certain the
concept will NEVER be applied to more than one "scope". That is, {topic}
should be the TOPIC ("ns", "extmark", "option", …) that acts on the scope(s)
(buf/win/tabpage/global), it should NOT be the scope. Instead the scope should
be a parameter (typically manifest as mutually-exclusive buf/win/… flags like
|nvim_get_option_value()|, or less commonly as a `scope: string` field like
|nvim_get_option_info2()|).

- Example: `nvim_get_keymap('v')` operates in a global context (first
  parameter is not a Buffer). The "get" verb indicates that it gets anything
  matching the given filter parameter. A "list" verb is unnecessary because
  `nvim_get_keymap('')` (empty filter) returns all items.
- Example: `nvim_buf_del_mark` acts on a `Buffer` object (the first parameter)
  and uses the "del" {verb}.


INTERFACE PATTERNS                                      *dev-api-patterns*

Prefer adding a single `nvim_{topic}_{verb}_…` interface for a given topic.

Example: >

    nvim_ns_add(
      ns_id: int,
      filter: {
        handle: integer (buf/win/tabpage id)
        scope: "global" | "win" | "buf" | "tabpage"
      }
    ): { ok: boolean }

    nvim_ns_get(
      ns_id: int,
      filter: {
        handle: integer (buf/win/tabpage id)
        scope: "global" | "win" | "buf" | "tabpage"
      }
    ): { ids: int[] }

    nvim_ns_del(
      ns_id: int,
      filter: {
        handle: integer (buf/win/tabpage id)
        scope: "global" | "win" | "buf" | "tabpage"
      }
    ): { ok: boolean }


Anti-Example:

Creating separate `nvim_xx`, `nvim_buf_xx`, `nvim_win_xx`, and
`nvim_tabpage_xx`, functions all for the same `xx` topic, requires 4x the
amount of documentation, tests, boilerplate, and interfaces, which users must
comprehend, maintainers must maintain, etc. Thus the following is NOT
recommended (compare these 12(!) functions to the above 3 functions): >

    nvim_add_ns(…)
    nvim_buf_add_ns(…)
    nvim_win_add_ns(…)
    nvim_tabpage_add_ns(…)
    nvim_del_ns(…)
    nvim_buf_del_ns(…)
    nvim_win_del_ns(…)
    nvim_tabpage_del_ns(…)
    nvim_get_ns(…)
    nvim_buf_get_ns(…)
    nvim_win_get_ns(…)
    nvim_tabpage_get_ns(…)

API-CLIENT                                              *dev-api-client*

                                                        *api-client*
API clients wrap the Nvim |API| to provide idiomatic "SDKs" for their
respective platforms (see |jargon|). You can build a new API client for your
favorite platform or programming language.

List of API clients:
    https://github.com/neovim/neovim/wiki/Related-projects#api-clients

                                                        *node-client* *pynvim*
These clients can be considered the "reference implementation" for API clients:
- https://github.com/neovim/node-client
- https://github.com/neovim/pynvim

Standard Features ~

- API clients exist to hide msgpack-rpc details. The wrappers can be
  automatically generated by reading the |api-metadata| from Nvim. |api-mapping|
- Clients should call |nvim_set_client_info()| after connecting, so users and
  plugins can detect the client by handling the |ChanInfo| event. This avoids
  the need for special variables or other client hints.
- Clients should handle |nvim_error_event| notifications, which will be sent
  if an async request to nvim was rejected or caused an error.

Package Naming ~

API client packages should NOT be named something ambiguous like "neovim" or
"python-client".  Use "nvim" as a prefix/suffix to some other identifier
following ecosystem conventions.

For example, Python packages tend to have "py" in the name, so "pynvim" is
a good name: it's idiomatic and unambiguous. If the package is named "neovim",
it confuses users, and complicates documentation and discussions.

Examples of API-client package names:
- ✅ OK: nvim-racket
- ✅ OK: pynvim
- ❌ NO: python-client
- ❌ NO: neovim_

API client implementation guidelines ~

- Separate the transport layer from the rest of the library. |rpc-connecting|
- Use a MessagePack library that implements at least version 5 of the
  MessagePack spec, which supports the BIN and EXT types used by Nvim.
- Use a single-threaded event loop library/pattern.
- Use a fiber/coroutine library for the language being used for implementing
  a client. These greatly simplify concurrency and allow the library to
  expose a blocking API on top of a non-blocking event loop without the
  complexity that comes with preemptive multitasking.
- Don't assume anything about the order of responses to RPC requests.
- Clients should expect requests, which must be handled immediately because
  Nvim is blocked while waiting for the client response.
- Clients should expect notifications, but these can be handled "ASAP" (rather
  than immediately) because they won't block Nvim.
- For C/C++ projects, consider libmpack instead of the msgpack.org library.
    https://github.com/libmpack/libmpack/
  libmpack is small (no dependencies, can inline into your C/C++ project) and
  efficient (no allocations). It also implements msgpack-RPC, the protocol
  required by Nvim.
    https://github.com/msgpack-rpc/msgpack-rpc


EXTERNAL UI                                             *dev-ui*

External UIs should be aware of the |api-contract|. In particular, future
versions of Nvim may add new items to existing events. The API is strongly
backwards-compatible, but clients must not break if new (optional) fields are
added to existing events.

Standard Features ~

External UIs are expected to implement these common features:

- Call |nvim_set_client_info()| after connecting, so users and plugins can
  detect the UI by handling the |ChanInfo| event. This avoids the need for
  special variables and UI-specific config files (gvimrc, macvimrc, …).
- Cursor style (shape, color) should conform to the 'guicursor' properties
  delivered with the mode_info_set UI event.
- Send the ALT/META ("Option" on macOS) key as a |<M-| chord.
- Send the "super" key (Windows key, Apple key) as a |<D-| chord.
- Avoid mappings that conflict with the Nvim keymap-space; GUIs have many new
  chords (<C-,> <C-Enter> <C-S-x> <D-x>) and patterns ("shift shift") that do
  not potentially conflict with Nvim defaults, plugins, etc.
- Consider the "option_set" |ui-global| event as a hint for other GUI
  behaviors. Various UI-related options ('guifont', 'ambiwidth', …) are
  published in this event. See also "mouse_on", "mouse_off".
- UIs generally should NOT set |$NVIM_APPNAME| (unless explicitly requested by
  the user).
- Support the text decorations/attributes given by |ui-event-hl_attr_define|.
  The "url" attr should be presented as a clickable hyperlink.


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