feat: ignore swapfile for running Nvim processes #25336

Problem:
The swapfile "E325: ATTENTION" dialog is displayed when editing a file
already open in another (running) Nvim. Usually this behavior is
annoying and irrelevant:
- "Recover" and the other options ("Open readonly", "Quit", "Abort") are
  almost never wanted.
- swapfiles are less relevant for "multi-Nvim" since 'autoread' is
  enabled by default.
  - Even less relevant if user enables 'autowrite'.

Solution:
Define a default SwapExists handler which does the following:
1. If the swapfile is owned by a running Nvim process, automatically
   chooses "(E)dit anyway" (caveat: this creates a new, extra swapfile,
   which is mostly harmless and ignored except by `:recover` or `nvim -r`.
2. Shows a 1-line "ignoring swapfile..." message.
3. Users can disable the default SwapExists handler via `autocmd! nvim_swapfile`.

1 files changed, 4 insertions, 3 deletions

diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 3cd5ea13f6..c41237b862 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -2276,12 +2276,13 @@ v:stderr	|channel-id| corresponding to stderr. The value is always 2;
 			:call chansend(v:stderr, "error: toaster empty\n")
 <
 					*v:swapname* *swapname-variable*
-v:swapname	Only valid when executing |SwapExists| autocommands: Name of
-		the swap file found.  Read-only.
+v:swapname	Name of the swapfile found.
+		Only valid during |SwapExists| event.
+		Read-only.
 
 					*v:swapchoice* *swapchoice-variable*
 v:swapchoice	|SwapExists| autocommands can set this to the selected choice
-		for handling an existing swap file:
+		for handling an existing swapfile:
 			'o'	Open read-only
 			'e'	Edit anyway
 			'r'	Recover