vim-patch:8.1.1205: BufReadPre may move the cursor #9980

Problem:    A BufReadPre autocommand may cause the cursor to move.
Solution:   Restore the cursor position after executing the autocommand,
            unless the autocommand moved it. (Christian Brabandt,
            closes vim/vim#4302, closes vim/vim#4294)
a68e595909

src/nvim/buffer_defs.h

@@ -999,6 +999,16 @@ typedef struct {
                                           .relative = 0, .external = false, \
                                           .focusable = true })
 
+// Structure to store last cursor position and topline.  Used by check_lnums()
+// and reset_lnums().
+typedef struct
+{
+  int w_topline_save;   // original topline value
+  int w_topline_corr;   // corrected topline value
+  pos_T w_cursor_save;  // original cursor position
+  pos_T w_cursor_corr;  // corrected cursor position
+} pos_save_T;
+
 /// Structure which contains all information that belongs to a window.
 ///
 /// All row numbers are relative to the start of the window, except w_winrow.
@@ -1091,17 +1101,18 @@ struct window_S {
   colnr_T w_skipcol;                /* starting column when a single line
                                        doesn't fit in the window */
 
-  /*
-   * Layout of the window in the screen.
-   * May need to add "msg_scrolled" to "w_winrow" in rare situations.
-   */
-  int w_winrow;                     /* first row of window in screen */
-  int w_height;                     /* number of rows in window, excluding
-                                       status/command line(s) */
-  int w_status_height;              /* number of status lines (0 or 1) */
-  int w_wincol;                     /* Leftmost column of window in screen. */
-  int w_width;                      /* Width of window, excluding separation. */
-  int w_vsep_width;                 /* Number of separator columns (0 or 1). */
+  //
+  // Layout of the window in the screen.
+  // May need to add "msg_scrolled" to "w_winrow" in rare situations.
+  //
+  int w_winrow;                     // first row of window in screen
+  int w_height;                     // number of rows in window, excluding
+                                    // status/command line(s)
+  int w_status_height;              // number of status lines (0 or 1)
+  int w_wincol;                     // Leftmost column of window in screen.
+  int w_width;                      // Width of window, excluding separation.
+  int w_vsep_width;                 // Number of separator columns (0 or 1).
+  pos_save_T w_save_cursor;         // backup of cursor pos and topline
 
   // inner size of window, which can be overridden by external UI
   int w_height_inner;

src/nvim/fileio.c

@@ -7072,6 +7072,8 @@ static bool apply_autocmds_group(event_T event, char_u *fname, char_u *fname_io,
     do_cmdline(NULL, getnextac, (void *)&patcmd,
                DOCMD_NOWAIT|DOCMD_VERBOSE|DOCMD_REPEAT);
 
+    reset_lnums();  // restore cursor and topline, unless they were changed
+
     if (eap != NULL) {
       (void)set_cmdarg(NULL, save_cmdarg);
       set_vim_var_nr(VV_CMDBANG, save_cmdbang);

src/nvim/testdir/test_autocmd.vim

@@ -1425,6 +1425,50 @@ func Test_autocmd_once()
   call assert_fails('au WinNew * ++once ++once echo bad', 'E983:')
 endfunc
 
+func Test_autocmd_bufreadpre()
+  new
+  let b:bufreadpre = 1
+  call append(0, range(100))
+  w! XAutocmdBufReadPre.txt
+  autocmd BufReadPre <buffer> :let b:bufreadpre += 1
+  norm! 50gg
+  sp
+  norm! 100gg
+  wincmd p
+  let g:wsv1 = winsaveview()
+  wincmd p
+  let g:wsv2 = winsaveview()
+  " triggers BufReadPre, should not move the cursor in either window
+  " The topline may change one line in a large window.
+  edit
+  call assert_inrange(g:wsv2.topline - 1, g:wsv2.topline + 1, winsaveview().topline)
+  call assert_equal(g:wsv2.lnum, winsaveview().lnum)
+  call assert_equal(2, b:bufreadpre)
+  wincmd p
+  call assert_equal(g:wsv1.topline, winsaveview().topline)
+  call assert_equal(g:wsv1.lnum, winsaveview().lnum)
+  call assert_equal(2, b:bufreadpre)
+  " Now set the cursor position in an BufReadPre autocommand
+  " (even though the position will be invalid, this should make Vim reset the
+  " cursor position in the other window.
+  wincmd p
+  1
+  " won't do anything, but try to set the cursor on an invalid lnum
+  autocmd BufReadPre <buffer> :norm! 70gg
+  " triggers BufReadPre, should not move the cursor in either window
+  e
+  call assert_equal(1, winsaveview().topline)
+  call assert_equal(1, winsaveview().lnum)
+  call assert_equal(3, b:bufreadpre)
+  wincmd p
+  call assert_equal(g:wsv1.topline, winsaveview().topline)
+  call assert_equal(g:wsv1.lnum, winsaveview().lnum)
+  call assert_equal(3, b:bufreadpre)
+  close
+  close
+  call delete('XAutocmdBufReadPre.txt')
+endfunc
+
 " Tests for the following autocommands:
 " - FileWritePre	writing a compressed file
 " - FileReadPost	reading a compressed file

src/nvim/window.c

@@ -5963,16 +5963,40 @@ void check_lnums(int do_curwin)
 {
   FOR_ALL_TAB_WINDOWS(tp, wp) {
     if ((do_curwin || wp != curwin) && wp->w_buffer == curbuf) {
+      // save the original cursor position and topline
+      wp->w_save_cursor.w_cursor_save = wp->w_cursor;
+      wp->w_save_cursor.w_topline_save = wp->w_topline;
+
       if (wp->w_cursor.lnum > curbuf->b_ml.ml_line_count) {
         wp->w_cursor.lnum = curbuf->b_ml.ml_line_count;
       }
       if (wp->w_topline > curbuf->b_ml.ml_line_count) {
         wp->w_topline = curbuf->b_ml.ml_line_count;
       }
+
+      // save the corrected cursor position and topline
+      wp->w_save_cursor.w_cursor_corr = wp->w_cursor;
+      wp->w_save_cursor.w_topline_corr = wp->w_topline;
     }
   }
 }
 
+/// Reset cursor and topline to its stored values from check_lnums().
+/// check_lnums() must have been called first!
+void reset_lnums(void)
+{
+  FOR_ALL_TAB_WINDOWS(tp, wp) {
+    if (wp->w_buffer == curbuf) {
+      // Restore the value if the autocommand didn't change it.
+      if (equalpos(wp->w_save_cursor.w_cursor_corr, wp->w_cursor)) {
+        wp->w_cursor = wp->w_save_cursor.w_cursor_save;
+      }
+      if (wp->w_save_cursor.w_topline_corr == wp->w_topline) {
+        wp->w_topline = wp->w_save_cursor.w_topline_save;
+      }
+    }
+  }
+}
 
 /*
  * A snapshot of the window sizes, to restore them after closing the help