Add -P flag to display-popup for passthrough/non-blocking mode

Add a new -P flag to display-popup that makes the popup non-blocking:
keyboard input is passed through to the underlying pane instead of
being captured by the popup. This allows users to continue typing
while the popup is displayed.

Escape and Ctrl-C still close the popup in passthrough mode.

This is useful for toast-style notifications that should not steal
focus. Combined with -E, popups can auto-dismiss when their command
exits while allowing uninterrupted typing.

Example usage:
  display-popup -P -E "echo 'Task completed'; sleep 2"

cmd-display-menu.c

@@ -54,8 +54,8 @@ const struct cmd_entry cmd_display_popup_entry = {
 	.name = "display-popup",
 	.alias = "popup",
 
-	.args = { "Bb:Cc:d:e:Eh:kNs:S:t:T:w:x:y:", 0, -1, NULL },
-	.usage = "[-BCEkN] [-b border-lines] [-c target-client] "
+	.args = { "Bb:Cc:d:e:Eh:kNPs:S:t:T:w:x:y:", 0, -1, NULL },
+	.usage = "[-BCEkNP] [-b border-lines] [-c target-client] "
 		 "[-d start-directory] [-e environment] [-h height] "
 		 "[-s style] [-S border-style] " CMD_TARGET_PANE_USAGE
 		 " [-T title] [-w width] [-x position] [-y position] "
@@ -504,6 +504,11 @@ cmd_display_popup_exec(struct cmd *self, struct cmdq_item *item)
 			flags = 0;
 		flags |= POPUP_CLOSEANYKEY;
 	}
+	if (args_has(args, 'P')) {
+		if (flags == -1)
+			flags = 0;
+		flags |= POPUP_PASSTHROUGH;
+	}
 
 	if (modify) {
 		popup_modify(tc, title, style, border_style, lines, flags);

popup.c

@@ -153,6 +153,10 @@ popup_mode_cb(__unused struct client *c, void *data, u_int *cx, u_int *cy)
 	if (pd->md != NULL)
 		return (menu_mode_cb(c, pd->md, cx, cy));
 
+	/* In passthrough mode, keep cursor in underlying pane. */
+	if (pd->flags & POPUP_PASSTHROUGH)
+		return (NULL);
+
 	if (pd->border_lines == BOX_LINES_NONE) {
 		*cx = pd->px + pd->s.cx;
 		*cy = pd->py + pd->s.cy;
@@ -504,6 +508,14 @@ popup_key_cb(struct client *c, void *data, struct key_event *event)
 		return (0);
 	}
 
+	/* Passthrough mode: pass non-mouse keys to underlying pane. */
+	if ((pd->flags & POPUP_PASSTHROUGH) && !KEYC_IS_MOUSE(event->key)) {
+		/* Still allow Escape and Ctrl-C to close the popup. */
+		if (event->key == '\033' || event->key == ('c'|KEYC_CTRL))
+			return (1);
+		return (-1);
+	}
+
 	if (KEYC_IS_MOUSE(event->key)) {
 		if (pd->dragging != OFF) {
 			popup_handle_drag(c, pd, m);

server-client.c

@@ -2636,9 +2636,14 @@ server_client_handle_key(struct client *c, struct key_event *event)
 			case 1:
 				server_client_clear_overlay(c);
 				return (0);
+			case -1:
+				break; /* passthrough to underlying pane */
+			default:
+				server_client_clear_overlay(c);
+				break;
 			}
-		}
-		server_client_clear_overlay(c);
+		} else
+			server_client_clear_overlay(c);
 		if (c->prompt_string != NULL) {
 			if (status_prompt_key(c, event->key) == 0)
 				return (0);
@@ -2920,6 +2925,8 @@ server_client_reset_state(struct client *c)
 	if (c->overlay_draw != NULL) {
 		if (c->overlay_mode != NULL)
 			s = c->overlay_mode(c, c->overlay_data, &cx, &cy);
+		if (s == NULL && c->prompt_string == NULL)
+			s = wp->screen;
 	} else if (c->prompt_string == NULL)
 		s = wp->screen;
 	else
@@ -2948,7 +2955,7 @@ server_client_reset_state(struct client *c)
 				cy = tty->sy - 1;
 		}
 		cx = c->prompt_cursor;
-	} else if (c->overlay_draw == NULL) {
+	} else if (c->overlay_draw == NULL || s == wp->screen) {
 		cursor = 0;
 		tty_window_offset(tty, &ox, &oy, &sx, &sy);
 		if (wp->xoff + s->cx >= ox && wp->xoff + s->cx <= ox + sx &&

tmux.1

@@ -7020,7 +7020,7 @@ forwards any input read from stdin to the empty pane given by
 .Ar target-pane .
 .Tg popup
 .It Xo Ic display-popup
-.Op Fl BCEkN
+.Op Fl BCEkNP
 .Op Fl b Ar border-lines
 .Op Fl c Ar target-client
 .Op Fl d Ar start-directory
@@ -7053,6 +7053,7 @@ Only the
 .Fl EE ,
 .Fl K ,
 .Fl N ,
+.Fl P ,
 .Fl s ,
 and
 .Fl S
@@ -7073,6 +7074,15 @@ allows any key to dismiss the popup instead of only
 .Ql Escape
 or
 .Ql C-c .
+.Fl P
+makes the popup non-blocking: keyboard input is sent to the underlying pane
+instead of the popup, allowing the user to continue typing while the popup
+is displayed.
+.Ql Escape
+and
+.Ql C-c
+will still close the popup.
+This is useful for toast-style notifications that should not steal focus.
 .Pp
 .Fl x
 and

tmux.h

@@ -3607,6 +3607,7 @@ int		 menu_key_cb(struct client *, void *, struct key_event *);
 #define POPUP_CLOSEEXITZERO 0x2
 #define POPUP_INTERNAL 0x4
 #define POPUP_CLOSEANYKEY 0x8
+#define POPUP_PASSTHROUGH 0x10
 typedef void (*popup_close_cb)(int, void *);
 typedef void (*popup_finish_edit_cb)(char *, size_t, void *);
 int		 popup_display(int, enum box_lines, struct cmdq_item *, u_int,