patch 8.1.0974: cannot switch from terminal window to previous tabpage

Problem:    Cannot switch from terminal window to previous tabpage.
Solution:   Make CTRL-W gT move to previous tabpage.

runtime/doc/terminal.txt

@@ -81,6 +81,7 @@ Special in the terminal window:			*CTRL-W_.*  *CTRL-W_N*
 			evaluating an expression.
 	CTRL-W CTRL-C	ends the job, see below |t_CTRL-W_CTRL-C|
 	CTRL-W gt	go to next tabpage, same as `gt`
+	CTRL-W gT	go to previous tabpage, same as `gT`
 
 See option 'termwinkey' for specifying another key instead of CTRL-W that
 will work like CTRL-W.  However, typing 'termwinkey' twice sends 'termwinkey'

src/testdir/test_terminal.vim

@@ -1569,10 +1569,14 @@ func Test_terminal_termwinsize_mininmum()
 endfunc
 
 func Test_terminal_termwinkey()
-  call assert_equal(1, winnr('$'))
-  let thiswin = win_getid()
-  tabnew
+  " make three tabpages, terminal in the middle
+  0tabnew
   tabnext
+  tabnew
+  tabprev
+  call assert_equal(1, winnr('$'))
+  call assert_equal(2, tabpagenr())
+  let thiswin = win_getid()
 
   let buf = Run_shell_in_terminal({})
   let termwin = bufwinid(buf)
@@ -1582,11 +1586,16 @@ func Test_terminal_termwinkey()
   call feedkeys("\<C-W>w", 'tx')
   call assert_equal(termwin, win_getid())
 
-  let tnr = tabpagenr()
   call feedkeys("\<C-L>gt", "xt")
-  call assert_notequal(tnr, tabpagenr())
+  call assert_equal(3, tabpagenr())
+  tabprev
+  call assert_equal(2, tabpagenr())
+  call assert_equal(termwin, win_getid())
+
+  call feedkeys("\<C-L>gT", "xt")
+  call assert_equal(1, tabpagenr())
   tabnext
-  call assert_equal(tnr, tabpagenr())
+  call assert_equal(2, tabpagenr())
   call assert_equal(termwin, win_getid())
 
   let job = term_getjob(buf)
@@ -1596,6 +1605,8 @@ func Test_terminal_termwinkey()
   set termwinkey&
   tabnext
   tabclose
+  tabprev
+  tabclose
 endfunc
 
 func Test_terminal_out_err()

src/version.c

@@ -779,6 +779,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    974,
 /**/
     973,
 /**/

src/window.c

@@ -87,10 +87,7 @@ do_window(
 #endif
     char_u	cbuf[40];
 
-    if (Prenum == 0)
-	Prenum1 = 1;
-    else
-	Prenum1 = Prenum;
+    Prenum1 = Prenum == 0 ? 1 : Prenum;
 
 #ifdef FEAT_CMDWIN
 # define CHECK_CMDWIN \
@@ -588,6 +585,10 @@ wingotofile:
 			goto_tabpage((int)Prenum);
 			break;
 
+		    case 'T':	    // CTRL-W gT: go to previous tab page
+			goto_tabpage(-(int)Prenum1);
+			break;
+
 		    default:
 			beep_flush();
 			break;