diff --git a/cmd-split-window.c b/cmd-split-window.c
index 5ef5425fb..f56aaac96 100644
--- a/cmd-split-window.c
+++ b/cmd-split-window.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: cmd-split-window.c,v 1.142 2026/07/13 10:03:27 nicm Exp $ */
+/* $OpenBSD: cmd-split-window.c,v 1.143 2026/07/13 16:07:47 nicm Exp $ */
 
 /*
  * Copyright (c) 2009 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -33,13 +33,15 @@
 #define SPLIT_WINDOW_TEMPLATE "#{session_name}:#{window_index}.#{pane_index}"
 
 static enum cmd_retval	cmd_split_window_exec(struct cmd *, struct cmdq_item *);
+static void		cmd_split_window_mouse_resize(struct client *,
+			    struct mouse_event *);
 
 const struct cmd_entry cmd_new_pane_entry = {
 	.name = "new-pane",
 	.alias = "newp",
 
-	.args = { "bB:c:de:EfF:hIkl:Lm:p:PR:s:S:t:T:vWx:X:y:Y:Z", 0, -1, NULL },
-	.usage = "[-bdefhIklPvWZ] [-B border-lines] "
+	.args = { "bB:c:de:EfF:hIkl:LMm:p:PR:s:S:t:T:vWx:X:y:Y:Z", 0, -1, NULL },
+	.usage = "[-bdefhIklMPvWZ] [-B border-lines] "
 		 "[-c start-directory] [-e environment] "
 		 "[-F format] [-l size] [-m message] [-p percentage] "
 		 "[-s style] [-S active-border-style] "
@@ -85,6 +87,7 @@ cmd_split_window_exec(struct cmd *self, struct cmdq_item *item)
 	struct layout_cell	*lc = NULL;
 	struct event_payload	*ep;
 	struct cmd_find_state	 fs;
+	struct key_event	*event = cmdq_get_event(item);
 	int			 input, empty, is_floating, flags = 0;
 	const char		*template, *style, *value;
 	char			*cause = NULL, *cp, *title;
@@ -100,6 +103,11 @@ cmd_split_window_exec(struct cmd *self, struct cmdq_item *item)
 		flags |= SPAWN_SPLIT;
 	}
 
+	if (args_has(args, 'M') && is_floating) {
+		if (event == NULL || !event->m.valid || tc == NULL)
+			return (CMD_RETURN_NORMAL);
+	}
+
 	if (is_floating)
 		flags |= SPAWN_FLOATING;
 	if (args_has(args, 'h'))
@@ -253,6 +261,12 @@ cmd_split_window_exec(struct cmd *self, struct cmdq_item *item)
 	}
 	server_redraw_session(s);
 
+	if (args_has(args, 'M') && is_floating) {
+		tc->tty.mouse_last_pane = new_wp->id;
+		tc->tty.mouse_drag_update = cmd_split_window_mouse_resize;
+		cmd_split_window_mouse_resize(tc, &event->m);
+	}
+
 	if (args_has(args, 'P')) {
 		if ((template = args_get(args, 'F')) == NULL)
 			template = SPLIT_WINDOW_TEMPLATE;
@@ -300,3 +314,73 @@ fail:
 	return (CMD_RETURN_ERROR);
 
 }
+
+static void
+cmd_split_window_mouse_resize(struct client *c, struct mouse_event *m)
+{
+	struct window_pane	*wp;
+	struct window		*w;
+	struct layout_cell	*lc;
+	enum pane_lines		 lines;
+	u_int			 sx, sy;
+	int			 x, y, xoff, yoff, border;
+
+	if (c->tty.mouse_last_pane == -1)
+		return;
+	wp = window_pane_find_by_id(c->tty.mouse_last_pane);
+	if (wp == NULL || !window_pane_is_floating(wp)) {
+		c->tty.mouse_drag_update = NULL;
+		return;
+	}
+	w = wp->window;
+	lc = wp->layout_cell;
+
+	x = m->x + m->ox;
+	y = m->y + m->oy;
+	if (m->statusat == 0 && y >= (int)m->statuslines)
+		y -= m->statuslines;
+	else if (m->statusat > 0 && y >= m->statusat)
+		y = m->statusat - 1;
+
+	lines = window_pane_get_pane_lines(wp);
+	border = (lines != PANE_LINES_NONE);
+
+	if (x >= (int)c->tty.mouse_drag_x) {
+		xoff = c->tty.mouse_drag_x + border;
+		sx = x - c->tty.mouse_drag_x + 1;
+	} else {
+		sx = c->tty.mouse_drag_x - x + 1;
+		xoff = c->tty.mouse_drag_x - sx + 1;
+		if (border)
+			xoff++;
+	}
+	if (y >= (int)c->tty.mouse_drag_y) {
+		yoff = c->tty.mouse_drag_y + border;
+		sy = y - c->tty.mouse_drag_y + 1;
+	} else {
+		sy = c->tty.mouse_drag_y - y + 1;
+		yoff = c->tty.mouse_drag_y - sy + 1;
+		if (border)
+			yoff++;
+	}
+
+	if (border) {
+		if (sx <= 2)
+			sx = PANE_MINIMUM;
+		else
+			sx -= 2;
+		if (sy <= 2)
+			sy = PANE_MINIMUM;
+		else
+			sy -= 2;
+	}
+	if (sx < PANE_MINIMUM)
+		sx = PANE_MINIMUM;
+	if (sy < PANE_MINIMUM)
+		sy = PANE_MINIMUM;
+
+	layout_set_size(lc, sx, sy, xoff, yoff);
+	layout_fix_panes(w, NULL);
+	server_redraw_window(w);
+	server_redraw_window_borders(w);
+}
diff --git a/key-bindings.c b/key-bindings.c
index 92d417c5b..359a7b5cb 100644
--- a/key-bindings.c
+++ b/key-bindings.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: key-bindings.c,v 1.186 2026/07/07 12:30:36 nicm Exp $ */
+/* $OpenBSD: key-bindings.c,v 1.187 2026/07/13 16:07:47 nicm Exp $ */
 
 /*
  * Copyright (c) 2007 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -496,10 +496,10 @@ key_bindings_init(void)
 
 		/* Mouse button 1 down on pane. */
 		"bind -n MouseDown1Pane { select-pane -t=; send -M }",
-		"bind -n C-MouseDown1Pane { swap-pane -s@ }",
 
 		/* Mouse button 1 drag on pane. */
 		"bind -n MouseDrag1Pane { if -F '#{||:#{pane_in_mode},#{mouse_any_flag}}' { send -M } { copy-mode -M } }",
+		"bind -n C-MouseDrag1Pane { new-pane -M }",
 		"bind -n M-MouseDrag1Pane { move-pane -M }",
 
 		/* Mouse wheel up on pane. */
diff --git a/server-client.c b/server-client.c
index ceb234f5a..3fb7813ca 100644
--- a/server-client.c
+++ b/server-client.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: server-client.c,v 1.488 2026/07/13 15:03:03 nicm Exp $ */
+/* $OpenBSD: server-client.c,v 1.489 2026/07/13 16:07:47 nicm Exp $ */
 
 /*
  * Copyright (c) 2009 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -1169,6 +1169,10 @@ have_event:
 		 * the scrollbar, store the relative position in the slider
 		 * where the user grabbed.
 		 */
+		if (c->tty.mouse_drag_flag == 0) {
+			c->tty.mouse_drag_x = px;
+			c->tty.mouse_drag_y = py;
+		}
 		c->tty.mouse_drag_flag = MOUSE_BUTTONS(b) + 1;
 
 		/* Only change pane if not already dragging a pane border. */
diff --git a/tmux.1 b/tmux.1
index 96ed453d1..025236b40 100644
--- a/tmux.1
+++ b/tmux.1
@@ -1,4 +1,4 @@
-.\" $OpenBSD: tmux.1,v 1.1135 2026/07/13 15:03:03 nicm Exp $
+.\" $OpenBSD: tmux.1,v 1.1136 2026/07/13 16:07:47 nicm Exp $
 .\"
 .\" Copyright (c) 2007 Nicholas Marriott <nicholas.marriott@gmail.com>
 .\"
@@ -3616,7 +3616,7 @@ but a different format may be specified with
 .Fl F .
 .Tg newp
 .It Xo Ic new\-pane
-.Op Fl bdefhIkLPvWZ
+.Op Fl bdefhIkLMPvWZ
 .Op Fl B Ar border\-lines
 .Op Fl c Ar start\-directory
 .Op Fl e Ar environment
@@ -3662,6 +3662,11 @@ to specify a percentage of the window size.
 .Fl B
 sets the pane border lines for floating panes; see
 .Ic pane\-border\-lines .
+.Fl M
+may be used when bound to a mouse drag key; the new floating pane is resized to
+follow the mouse drag.
+It has no effect with
+.Fl L .
 .Pp
 The
 .Fl L
diff --git a/tmux.h b/tmux.h
index 05068a1ef..6930eaf53 100644
--- a/tmux.h
+++ b/tmux.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: tmux.h,v 1.1400 2026/07/13 15:03:03 nicm Exp $ */
+/* $OpenBSD: tmux.h,v 1.1401 2026/07/13 16:07:47 nicm Exp $ */
 
 /*
  * Copyright (c) 2007 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -1780,6 +1780,8 @@ struct tty {
 	u_int		 mouse_last_y;
 	u_int		 mouse_last_b;
 	int		 mouse_drag_flag;
+	u_int		 mouse_drag_x;
+	u_int		 mouse_drag_y;
 	int		 mouse_scrolling_flag;
 	int		 mouse_slider_mpos;
 	int              mouse_last_pane;