fix(nushell): adapt the completer to the 0.116 contract

Nushell 0.116 runs an external completer on a background worker
without the terminal unless the registered closure reaches a
command carrying the @interactive attribute. Interactivity is
seen through the closure to the command it calls, so wrap the
completion logic in an @interactive entry point and register a
closure that ends with the call to it.

Read the named `place` input instead of the deprecated
positional `spans`; $place.command carries the same token list.

An @interactive completer cannot decline with null (null reads
as a dismissed picker), so request the fallback explicitly when
the `**` trigger is absent, keeping plain file completion
working. The previous external completer is still called when
the trigger is absent.

The script now requires Nushell 0.116+, since @interactive is a
parse error on older versions.

Fixes #4932

shell/completion.nu

@@ -8,6 +8,15 @@
 # An implementation of completion.nu
 # This loads FZF as a Nushell External Completer
 # https://www.nushell.sh/cookbook/external_completers.html
+#
+# Requires Nushell 0.116.0 or newer.
+#
+# On 0.116+, the registered completer closure must reach a command carrying the
+# `@interactive` attribute: interactivity is seen through the closure to that
+# command (a closure cannot carry the attribute itself), and that is what makes
+# the completion run on the REPL thread with the terminal, so fzf can take it.
+# A closure that does not reach such a command runs on a background worker
+# without the terminal, where fzf cannot run.
 
 
 # --- Default Environment Variables ---
@@ -367,9 +376,9 @@ def _fzf_complete_kill [query: string] {
 
 # --- Main FZF External Completer ---
 
-# This function is registered with Nushell's external completion system.
-# It gets called when Tab is pressed.
-let fzf_external_completer = {|spans|
+# Main completion logic. It receives the token list of the command being
+# completed (the legacy `spans`). Called directly, so no attribute is needed.
+def __fzf_completer_logic [spans] {
   let trigger: string = $env.FZF_COMPLETION_TRIGGER? | default '**'
 
   if ($trigger | is-empty)     { return null } # Cannot work with empty trigger
@@ -446,6 +455,29 @@ let fzf_external_completer = {|spans|
   }
 }
 
+# Nushell 0.116+ entry point. The registered closure must END with a call to
+# this command for the completion to run on the REPL thread with the terminal.
+#
+# It declares `place` (a named input) rather than the legacy `spans`, so the
+# completer never goes through the deprecated positional bridge. `$place.command`
+# is the token list of the command being completed, the same value the old
+# `spans` held (with a trailing "" when the cursor sits on an empty slot).
+@interactive
+def __fzf_external_completer [place] {
+  let result = (__fzf_completer_logic $place.command)
+  if $result != null {
+    $result
+  } else {
+    # Fall back to the previously configured external completer, if any.
+    let previous = ($env.__fzf_previous_completer? | default null)
+    let previous_result = if $previous != null { do $previous $place } else { null }
+    # An @interactive completer cannot decline with null: null reads as a
+    # dismissed picker with an empty, final answer. Request the fallback
+    # explicitly so file completion still runs when the trigger is absent.
+    if $previous_result != null { $previous_result } else { {completions: [], fallback: true} }
+  }
+}
+
 # --- WRAPPER AND REGISTRATION ---
 
 # Guard against re-sourcing: wrapping the completer multiple times would
@@ -455,25 +487,15 @@ if ($env.__fzf_completer_registered? | default false) != true {
   # Get the currently configured external completer, if any exists
   let previous_external_completer = $env.config? | get completions? | get external? | get completer?
 
-  # Define the new wrapper completer
-  let fzf_wrapper_completer = {|spans|
-    # 1. Try the FZF completer logic first
-    let fzf_result = do $fzf_external_completer $spans
+  # Stash it for `__fzf_external_completer`: a def cannot capture a local
+  # variable, and the fallback to the previous completer happens inside it.
+  $env.__fzf_previous_completer = $previous_external_completer
 
-    # 2. If FZF returned a result (a list, even an empty one), return it.
-    #    `null` means FZF didn't handle it because the trigger wasn't present.
-    if $fzf_result != null {
-      $fzf_result
-    } else {
-      # 3. FZF didn't handle it, so call the previous completer (if it exists).
-      if $previous_external_completer != null {
-        do $previous_external_completer $spans
-      } else {
-        # 4. No previous completer, and FZF didn't handle it. Return null.
-        null
-      }
-    }
-  }
+  # The registered closure must END with a call to an `@interactive` command so
+  # that Nushell 0.116+ runs the completion on the REPL thread with the terminal
+  # (it inspects the last pipeline element of the closure). Declaring `place`
+  # instead of `spans` keeps it off the deprecated positional bridge.
+  let fzf_wrapper_completer = {|place| __fzf_external_completer $place }
 
   # Register the new wrapper completer
   # This ensures external completions are enabled and sets our wrapper.