Problem:
vim.diagnostic.set() defers extmark position computation for an
unloaded buffer via a once=true BufRead autocmd, registering a new one
on every call without replacing the previous one. Each pending autocmd
also retains that call's diagnostics.
Solution:
Instead of registering an autocmd per set() call, register a single
static BufRead autocmd that computes positions from the diagnostic
cache for any buffer with cached diagnostics when it is read. This
removes the per-call registration entirely (nothing left to
accumulate) and means diagnostics cleared while the buffer was
unloaded no longer produce stale extmarks.
Problem:
`nvim_create_autocmd` is too verbose and its `callback` requires extra
"nesting".
Solution:
Introduce `nvim_on`. Start using it internally. Then we can get a feel
for how it should look before making it public.
Extract the diagnostic implementation from
runtime/lua/vim/diagnostic.lua into focused internal modules covering
config, display, float rendering, jump/list helpers, namespace and
storage management, severity/shared utilities, and statusline support.
Move the builtin handlers into runtime/lua/vim/diagnostic/handlers/ and
keep runtime/lua/vim/diagnostic.lua as the public facade that lazily
dispatches to the split modules. This preserves the external
vim.diagnostic API while making the implementation easier to navigate
and reason about.
AI-assisted: Codex
