patch 9.0.0445: when opening/closing window text moves up/down

Problem:    When opening/closing window text moves up/down.
Solution:   Add the 'splitscroll' option.  When off text will keep its
            position as much as possible.

Author: luukvbaal

runtime/doc/options.txt

@@ -7499,6 +7499,18 @@ A jump table for the options with a short description can be found at |Q_op|.
 	When on, splitting a window will put the new window right of the
 	current one. |:vsplit|
 
+			*'splitscroll'* *'spsc'* *'nosplitscroll'* *'nospsc'*
+'splitscroll' 'spsc'	boolean	(default on)
+			global
+	The value of this option determines the scroll behavior when opening,
+	closing or resizing horizontal splits. When "on", splitting a window
+	horizontally will keep the same relative cursor position in the old and
+	new window, as well windows that are resized. When "off", scrolling
+	will be avoided to stabilize the window content. Instead, the cursor
+	position will be changed when necessary. In this case, the jumplist
+	will be populated with the previous cursor position. Scrolling cannot
+	be guaranteed to be avoided when 'wrap' is enabled.
+
 			   *'startofline'* *'sol'* *'nostartofline'* *'nosol'*
 'startofline' 'sol'	boolean	(default on)
 			global

runtime/doc/quickref.txt

@@ -919,6 +919,7 @@ Short explanation of each option:		*option-list*
 'spellsuggest'	  'sps'     method(s) used to suggest spelling corrections
 'splitbelow'	  'sb'	    new window from split is below the current one
 'splitright'	  'spr'     new window is put right of the current one
+'splitscroll'	  'spsc'    determines scroll behavior when splitting windows
 'startofline'	  'sol'     commands move cursor to first non-blank in line
 'statusline'	  'stl'     custom format for the status line
 'suffixes'	  'su'	    suffixes that are ignored with multiple match

runtime/optwin.vim

@@ -515,6 +515,8 @@ call <SID>AddOption("splitbelow", gettext("a new window is put below the current
 call <SID>BinOptionG("sb", &sb)
 call <SID>AddOption("splitright", gettext("a new window is put right of the current one"))
 call <SID>BinOptionG("spr", &spr)
+call <SID>AddOption("splitscroll", gettext("determines scroll behavior when spliting windows"))
+call <SID>BinOptionG("spsc", &spsc)
 call <SID>AddOption("scrollbind", gettext("this window scrolls together with other bound windows"))
 call append("$", "\t" .. s:local_to_window)
 call <SID>BinOptionL("scb")

src/move.c

@@ -981,7 +981,8 @@ curs_columns(
     /*
      * First make sure that w_topline is valid (after moving the cursor).
      */
-    update_topline();
+    if (p_spsc)
+	update_topline();
 
     /*
      * Next make sure that w_cline_row is valid.

src/option.h

@@ -924,6 +924,7 @@ EXTERN char_u	*p_spo;		// 'spelloptions'
 EXTERN char_u	*p_sps;		// 'spellsuggest'
 #endif
 EXTERN int	p_spr;		// 'splitright'
+EXTERN int	p_spsc;		// 'splitscroll'
 EXTERN int	p_sol;		// 'startofline'
 EXTERN char_u	*p_su;		// 'suffixes'
 EXTERN char_u	*p_sws;		// 'swapsync'

src/optiondefs.h

@@ -2349,6 +2349,9 @@ static struct vimoption options[] =
     {"splitright",  "spr",  P_BOOL|P_VI_DEF,
 			    (char_u *)&p_spr, PV_NONE,
 			    {(char_u *)FALSE, (char_u *)0L} SCTX_INIT},
+    {"splitscroll", "spsc", P_BOOL,
+			    (char_u *)&p_spsc, PV_NONE,
+			    {(char_u *)TRUE, (char_u *)TRUE} SCTX_INIT},
     {"startofline", "sol",  P_BOOL|P_VI_DEF|P_VIM,
 			    (char_u *)&p_sol, PV_NONE,
 			    {(char_u *)TRUE, (char_u *)0L} SCTX_INIT},

src/structs.h

@@ -3570,6 +3570,8 @@ struct window_S
     int		w_winrow;	    // first row of window in screen
     int		w_height;	    // number of rows in window, excluding
 				    // status/command/winbar line(s)
+    int		w_prev_winrow;	    // previous winrow used for 'splitscroll'
+    int		w_prev_height;	    // previous height used for 'splitscroll'
 
     int		w_status_height;    // number of status lines (0 or 1)
     int		w_wincol;	    // Leftmost column of window in screen.

src/testdir/test_window_cmd.vim

@@ -1631,5 +1631,133 @@ func Test_win_equal_last_status()
   set laststatus&
 endfunc
 
+" Ensure no scrolling happens with 'nosplitscroll' with and without a
+" winbar, tabline, for each possible value of 'laststatus', 'scrolloff',
+" 'equalalways', and regardless of the cursor position.
+func Test_splitscroll_with_splits()
+  set nowrap
+  set nosplitscroll
+  let gui = has("gui_running")
+  inoremap c <cmd>:copen<CR>
+  for winbar in [0, 1]
+    for sb in [0, 1]
+      for ea in [0, 1]
+        for tab in [0, 1]
+          for so in [0, 5]
+            for ls in range(0, 2)
+              for pos in ["H", "M", "L"]
+              let tabline = (gui ? 0 : (tab ? 1 : 0))
+              let winbar_sb = (sb ? winbar : 0)
+              execute 'set scrolloff=' . so
+              execute 'set laststatus=' . ls
+              execute 'set ' . (ea ? 'equalalways' : 'noequalalways')
+              execute 'set ' . (sb ? 'splitbelow' : 'nosplitbelow')
+              execute tab ? 'tabnew' : ''
+              execute winbar ? 'nnoremenu 1.10 WinBar.Test :echo' : ''
+              call setline(1, range(1, 256))
+              execute 'norm gg' . pos
+              " No scroll for vertical split and quit
+              vsplit | quit
+              call assert_equal(1, line("w0"))
+
+              " No scroll for horizontal split
+              split | redraw! | wincmd k
+              call assert_equal(1, line("w0"))
+
+              " No scroll when resizing windows
+              resize +2
+              call assert_equal(1, line("w0"))
+              wincmd j
+              call assert_equal(win_screenpos(0)[0] - tabline - winbar_sb, line("w0"))
+
+              " No scroll when dragging statusline
+              call win_move_statusline(1, -3)
+              call assert_equal(win_screenpos(0)[0] - tabline - winbar_sb, line("w0"))
+              wincmd k
+              call assert_equal(1, line("w0"))
+
+              " No scroll when changing shellsize
+              set lines+=2
+              call assert_equal(1, line("w0"))
+              wincmd j
+              call assert_equal(win_screenpos(0)[0] - tabline - winbar_sb, line("w0"))
+              set lines-=2
+              call assert_equal(win_screenpos(0)[0] - tabline - winbar_sb, line("w0"))
+              wincmd k
+              call assert_equal(1, line("w0"))
+
+              " No scroll when equalizing windows
+              wincmd =
+              call assert_equal(1, line("w0"))
+              wincmd j
+              call assert_equal(win_screenpos(0)[0] - tabline - winbar_sb, line("w0"))
+              wincmd k
+              call assert_equal(1, line("w0"))
+
+              " No scroll in windows split multiple times
+              vsplit | split | 4wincmd w
+              call assert_equal(win_screenpos(0)[0] - tabline - winbar_sb, line("w0"))
+              1wincmd w | quit | wincmd l | split
+              call assert_equal(win_screenpos(0)[0] - tabline - winbar_sb, line("w0"))
+              wincmd j
+              call assert_equal(win_screenpos(0)[0] - tabline - winbar_sb, line("w0"))
+
+              " No scroll in small window
+              2wincmd w | only | 5split | wincmd k
+              call assert_equal(1, line("w0"))
+              wincmd j
+              call assert_equal(win_screenpos(0)[0] - tabline - winbar_sb, line("w0"))
+
+              " No scroll for vertical split
+              quit | vsplit | wincmd l
+              call assert_equal(1, line("w0"))
+              wincmd h
+              call assert_equal(1, line("w0"))
+
+              " No scroll in windows split and quit multiple times
+              quit | split | split | quit
+              call assert_equal(win_screenpos(0)[0] - tabline - winbar_sb, line("w0"))
+
+              " No scroll for new buffer
+              1wincmd w | only | copen | wincmd k
+              call assert_equal(1, line("w0"))
+              only
+              call assert_equal(1, line("w0"))
+              above copen | wincmd j
+              call assert_equal(win_screenpos(0)[0] - tabline, line("w0"))
+
+              " No scroll when opening cmdwin
+              only | norm ggLq:
+              call assert_equal(1, line("w0"))
+
+              " Scroll when cursor becomes invalid in insert mode
+              norm Lic
+              wincmd k | only
+              call assert_notequal(1, line("w0"))
+
+              " No scroll when topline not equal to 1
+              execute "norm gg5\<C-e>" | split | wincmd k
+              call assert_equal(6, line("w0"))
+              wincmd j
+              call assert_equal(5 + win_screenpos(0)[0] - tabline - winbar_sb, line("w0"))
+              only
+              endfor
+            endfor
+          endfor
+          tabonly!
+        endfor
+      endfor
+    endfor
+  endfor
+
+  tabnew | tabonly! | %bwipeout!
+  iunmap c
+  set wrap&
+  set scrolloff&
+  set splitbelow&
+  set laststatus&
+  set equalalways&
+  set splitscroll&
+endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -703,6 +703,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    445,
 /**/
     444,
 /**/