Problem:
There is no unified notion of a "user action".
Vim processes input by one-char-at-a-time, and mostly throws away any
hints it might gather about the user's action, with one exception: it
stores the last _edit_ action (the "redo buffer", encoded as
unstructured `["x][v][count]body` bytes).
Plugins can only observe individual keys (vim.on_key) and high-level
effects (TextChanged, CursorMoved).
Solution:
- Users can subscribe to `CmdAtom` events to handle any user action.
- Event is deferred; handlers cannot cancel or interfere with user
actions.
- Capture `CmdSpec` from the normal/insert/visual subsystems.
- typeahead/readahead stay unstructured (`buffheader_T`): they are key
streams, not commands.
- the redo/record buffers become `StringBuilder`: fewer
allocations/copies.
- Repurpose the input/redo engine to accept `CmdSpec` objects.
"atom": one repeatable unit of user input, as a resolved (post-mapping)
keysequence plus structured fields. Only user actions, not `:normal`,
API calls, or non-"t" `feedkeys`.
BREAKING: dot-repeat of an Insert session, replays the entire session
including cursor-moves (:help ins-repeat).
BREAKING: dot-repeat of a Visual operation, replays the selection
instead of operating on a fixed-size region.
Problem: Attach-time terminal probes cannot distinguish responses from
different attached UIs.
Solution: Identify the UI by RPC channel id in `TermResponse` and make
`vim.tty.request()` filter responses by channel.
Problem:
No way to handle a "tab moved" event.
Use-case: tabline plugins may cache tab labels, and need to know when to
invalidate their cache.
Solution:
Add a `TabMoved` event that triggers whenever tabs are reordered via `:tabmove`
or via mouse click-and-drag.
Problem:
No LuaLS types for event-data fields (ev.data). Types are only
documented ad hoc in scattered locations.
Solution:
Add runtime/lua/vim/_meta/events.lua defining vim.event.<name>.data
classes for events that provide ev.data. Reference the types from
each event's help in autocmd.txt, lsp.txt, and pack.txt.
