adds option to control whether tagstack is copied to new window

fixes: #5742

runtime/doc/options.txt

@@ -2356,6 +2356,11 @@ A jump table for the options with a short description can be found at |Q_op|.
 	NOTE: This option is reset when 'compatible' is set.
 	Also see 'preserveindent'.
 
+				*'copytagstack'* *'cptgst'* *'nocopytagstack'* *'nocptgst'*
+'copytagstack' 'cptgst'		boolean	(default: on)
+				global
+	Copy tag stack when splitting window.
+
 						*'cpoptions'* *'cpo'* *cpo*
 'cpoptions' 'cpo'	string	(Vim default: "aABceFsz",
 				 Vi default:  all flags, except "#{|&/\."

src/option.h

@@ -422,6 +422,7 @@ EXTERN char_u	*p_ofu;		// 'omnifunc'
 EXTERN char_u	*p_tsrfu;	// 'thesaurusfunc'
 #endif
 EXTERN int	p_ci;		// 'copyindent'
+EXTERN int	p_cptgst;	// 'copytagstack'
 #if defined(FEAT_GUI) && defined(MACOS_X)
 EXTERN int	*p_antialias;	// 'antialias'
 #endif

src/optiondefs.h

@@ -727,6 +727,9 @@ static struct vimoption options[] =
     {"copyindent",  "ci",   P_BOOL|P_VI_DEF|P_VIM,
 			    (char_u *)&p_ci, PV_CI, NULL, NULL,
 			    {(char_u *)FALSE, (char_u *)0L} SCTX_INIT},
+    {"copytagstack", "cptgst", P_BOOL|P_VIM,
+			    (char_u *)&p_cptgst, PV_NONE, NULL, NULL,
+			    {(char_u *)TRUE, (char_u *)TRUE} SCTX_INIT},
     {"cpoptions",   "cpo",  P_STRING|P_VIM|P_RALL|P_FLAGLIST,
 			    (char_u *)&p_cpo, PV_NONE, did_set_cpoptions, expand_set_cpoptions,
 			    {(char_u *)CPO_VI, (char_u *)CPO_VIM}

src/testdir/Make_all.mak

@@ -114,6 +114,7 @@ NEW_TESTS = \
 	test_compiler \
 	test_conceal \
 	test_const \
+	test_copytagstack \
 	test_cpoptions \
 	test_crash \
 	test_crypt \
@@ -398,6 +399,7 @@ NEW_TESTS_RES = \
 	test_comparators.res \
 	test_conceal.res \
 	test_const.res \
+	test_copytagstack.res \
 	test_cpoptions.res \
 	test_crash.res \
 	test_crypt.res \

src/testdir/test_copytagstack.vim

@@ -0,0 +1,46 @@
+" test 'copytagstack' option
+
+source check.vim
+source view_util.vim
+
+func Test_copytagstack()
+  call writefile(["int Foo;"], 'file.c', 'D')
+  call writefile(["Foo\tfile.c\t1"], 'Xtags', 'D')
+  set tags=Xtags
+
+  tag Foo
+
+  let nr0 = winnr()
+  call assert_equal(1, gettagstack(nr0)['length'])
+
+  split Xtext
+
+  let nr1 = winnr()
+  call assert_equal(1, gettagstack(nr1)['length'])
+
+  set tags&
+  bwipe
+endfunc
+
+func Test_nocopytagstack()
+  call writefile(["int Foo;"], 'file.c', 'D')
+  call writefile(["Foo\tfile.c\t1"], 'Xtags', 'D')
+  set tags=Xtags
+  set nocopytagstack
+
+  tag Foo
+
+  let nr0 = winnr()
+  call assert_equal(1, gettagstack(nr0)['length'])
+
+  split Xtext
+
+  let nr1 = winnr()
+  call assert_equal(0, gettagstack(nr1)['length'])
+
+  set tags&
+  set copytagstack&
+  bwipe
+endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab

src/window.c

@@ -1559,6 +1559,8 @@ win_init(win_T *newp, win_T *oldp, int flags UNUSED)
     }
 
     // copy tagstack and folds
+    if (p_cptgst)
+    {
 	for (i = 0; i < oldp->w_tagstacklen; i++)
 	{
 	    taggy_T	*tag = &newp->w_tagstack[i];
@@ -1570,6 +1572,7 @@ win_init(win_T *newp, win_T *oldp, int flags UNUSED)
 	}
 	newp->w_tagstackidx = oldp->w_tagstackidx;
 	newp->w_tagstacklen = oldp->w_tagstacklen;
+    }
 
     // Keep same changelist position in new window.
     newp->w_changelistidx = oldp->w_changelistidx;