Don't let move_current_tab move the tab beyond the main menu.

src/terminal/tab.c (get_tab_by_number): Assert that the returned
struct window * actually points to a struct window.

src/terminal/tab.c (move_current_tab): Keep tabs contiguous in the
stack of windows.  Obey "ui.tabs.wraparound".

src/terminal/terminal.h (struct terminal): Documented that tabs can
move in the stack.

src/terminal/window.c (assert_window_stacking): Check that the main
menu isn't between tabs.

src/terminal/tab.c

@@ -103,6 +103,12 @@ get_tab_by_number(struct terminal *term, int num)
 		num--;
 	}
 
+	/* Ensure that the return value actually points to a struct
+	 * window.  */
+	assertm((struct list_head *) win != &term->windows,
+	        "tab number out of range");
+	if_assert_failed return term->windows.next;
+
 	return win;
 }
 
@@ -301,22 +307,27 @@ move_current_tab(struct session *ses, int direction)
 
 	new_pos = term->current_tab + direction;
 
-	while (new_pos < 1 || new_pos > tabs)
+	if (get_opt_bool("ui.tabs.wraparound")) {
-		new_pos += new_pos < 1 ? tabs : -tabs;
+		while (new_pos < 0 || new_pos >= tabs)
-
+			new_pos += new_pos < 0 ? tabs : -tabs;
-	assert(0 < new_pos && new_pos <= tabs);
+	} else {
+		if (new_pos < 0)
+			new_pos = 0;
+		else if (new_pos >= tabs)
+			new_pos = tabs - 1;
+	}
+	assert(0 <= new_pos && new_pos < tabs);
 
+	/* This protects against tabs==1 and optimizes an unusual case.  */
 	if (new_pos == term->current_tab) return;
 
-	tab = get_tab_by_number(term, new_pos);
-
 	del_from_list(current_tab);
-
+	if (new_pos == 0) {
-	if (new_pos < term->current_tab) {
+		tab = get_tab_by_number(term, 0);
-		add_at_pos(tab, current_tab);
 	} else {
-		add_to_list_end(*tab, current_tab);
+		tab = get_tab_by_number(term, new_pos-1)->prev;
 	}
+	add_at_pos(tab, current_tab);
 
 	switch_to_tab(term, new_pos, tabs);
 }

src/terminal/terminal.h

@@ -73,9 +73,9 @@ struct terminal {
 	 * current window; and .prev is the bottom, covered by others.
 	 * - Dialogs or active menus are at the top.
 	 * - Next come all tabs (window.type == WINDOW_TAB).  The tab
-	 *   created last is at the top, and the tab created first is
+	 *   listed leftmost in the tab bar is at the top, and the tab
-	 *   at the bottom; but current_tab controls which tab is
+	 *   listed rightmost is at the bottom; but current_tab controls
-	 *   actually displayed.
+	 *   which tab is actually displayed.
 	 * - If the main menu is inactive, then it is at the very bottom,
 	 *   hidden under the tabs.
 	 * Call assert_window_stacking to verify this.

src/terminal/window.c

@@ -178,7 +178,7 @@ add_empty_window(struct terminal *term, void (*fn)(void *), void *data)
 void
 assert_window_stacking(struct terminal *term)
 {
-	int prev_type = WINDOW_NORMAL;
+	enum { WANT_ANY, WANT_TAB, WANT_NONE } want = WANT_ANY;
 	const struct window *win;
 	const struct window *main_menu_win;
 
@@ -186,10 +186,20 @@ assert_window_stacking(struct terminal *term)
 	main_menu_win = term->main_menu ? term->main_menu->win : NULL;
 
 	foreach (win, term->windows) {
-		if (win != main_menu_win) {
+		switch (want) {
-			if (prev_type == WINDOW_TAB)
+		case WANT_ANY:
+			if (win->type == WINDOW_TAB)
+				want = WANT_TAB;
+			break;
+		case WANT_TAB:
+			if (win == main_menu_win)
+				want = WANT_NONE;
+			else
 				assert(win->type == WINDOW_TAB);
-			prev_type = win->type;
+			break;
+		case WANT_NONE:
+			assert(0);
+			break;
 		}
 	}
 }