fix(lsp): ignore hover and signatureHelp responses on buffer change (#21121)

Language servers can take some time to respond to the
`textDocument/hover` and `textDocument/signatureHelp` messages. During
that time, the user could have already moved to another buffer. The
popup was always shown in the current buffer, which could be a different
one than the buffer for which the request was sent.

This was particularly annoying when moving to a buffer with a `BufLeave`
autocmd, as that autocmd was triggered when the hover popup was shown
for the original buffer.

Ignoring the response from these 2 messages if they are for a buffer
that is not the current one leads to less noise. The popup will only be
shown for the buffer for which it was requested.

A more robust solution could involve cancelling the hover/signatureHelp
request if the buffer changes so the language server can free its
resources. It could be implemented in the future.

runtime/lua/vim/lsp/handlers.lua

@@ -328,6 +328,10 @@ end
 function M.hover(_, result, ctx, config)
   config = config or {}
   config.focus_id = ctx.method
+  if api.nvim_get_current_buf() ~= ctx.bufnr then
+    -- Ignore result since buffer changed. This happens for slow language servers.
+    return
+  end
   if not (result and result.contents) then
     vim.notify('No information available')
     return
@@ -408,6 +412,10 @@ M['textDocument/implementation'] = location_handler
 function M.signature_help(_, result, ctx, config)
   config = config or {}
   config.focus_id = ctx.method
+  if api.nvim_get_current_buf() ~= ctx.bufnr then
+    -- Ignore result since buffer changed. This happens for slow language servers.
+    return
+  end
   -- When use `autocmd CompleteDone <silent><buffer> lua vim.lsp.buf.signature_help()` to call signatureHelp handler
   -- If the completion item doesn't have signatures It will make noise. Change to use `print` that can use `<silent>` to ignore
   if not (result and result.signatures and result.signatures[1]) then