'inccommand': preserve 'modified'

During the live preview, the buffer-local 'modified' flag
should not be changed.

src/nvim/ex_cmds.c

@@ -3142,6 +3142,7 @@ buf_T *do_sub(exarg_T *eap)
   pos_T old_cursor = curwin->w_cursor;
   int start_nsubs;
   int save_ma = 0;
+  int save_b_changed = curbuf->b_changed;
 
   if (!global_busy) {
     sub_nsubs = 0;
@@ -3971,6 +3972,8 @@ skip:
       }
     }
 
+    curbuf->b_changed = save_b_changed;  // preserve 'modified' during preview
+    // set_option_value((char_u *)"modified", 0L, NULL, OPT_LOCAL);
     preview_buf = show_sub(pat, sub, eap->line1, eap->line2, &matched_lines);
 
   } else if (*p_icm != NUL && eap->is_live) {
@@ -6014,9 +6017,8 @@ void set_context_in_sign_cmd(expand_T *xp, char_u *arg)
   }
 }
 
-/// Shows the effects of the current :substitute command being typed
+/// Shows the effects of the :substitute command being typed ('inccommand').
-/// ('inccommand'). If inccommand=split, shows a preview window then later
+/// If inccommand=split, shows a preview window and later restores the layout.
-/// restores the layout.
 static buf_T *show_sub(char_u *pat, char_u *sub, linenr_T line1, linenr_T line2,
                        MatchedLineVec *matched_lines)
   FUNC_ATTR_NONNULL_ALL
@@ -6050,7 +6052,7 @@ static buf_T *show_sub(char_u *pat, char_u *sub, linenr_T line1, linenr_T line2,
     buf_open_special(preview_buf ? bufnr : 0, "[Preview]", "incsub");
     buf_clear();
     preview_buf = curbuf;
-    set_option_value((char_u *)"bh", 0L, (char_u *)"hide", OPT_LOCAL);
+    set_option_value((char_u *)"bufhidden", 0L, (char_u *)"hide", OPT_LOCAL);
     bufnr = preview_buf->handle;
     curbuf->b_p_bl = false;
     curbuf->b_p_ma = true;

src/nvim/ex_getln.c

@@ -1594,6 +1594,7 @@ static int command_line_changed(CommandLineState *s)
   } else if (s->firstc == ':'
              && KeyTyped            // only if interactive
              && *p_icm != NUL       // 'inccommand' is set
+             && curbuf->b_p_ma      // buffer is modifiable
              && cmdline_star == 0   // not typing a password
              && cmd_is_live(ccline.cmdbuff)) {
     // process a "live" command ('inccommand')

test/functional/ui/inccommand_spec.lua

@@ -610,6 +610,30 @@ describe(":substitute, inccommand=split", function()
     screen:detach()
   end)
 
+  it("preserves 'modified' buffer flag", function()
+    execute("set nomodified")
+    feed(":%s/tw")
+    screen:expect([[
+      Inc substitution on           |
+      two lines                     |
+                                    |
+      {15:~                             }|
+      {15:~                             }|
+      {11:[No Name]                     }|
+      |2| two lines                 |
+      |4| two lines                 |
+                                    |
+      {15:~                             }|
+      {15:~                             }|
+      {15:~                             }|
+      {15:~                             }|
+      {10:[Preview]                     }|
+      :%s/tw^                        |
+    ]])
+    feed([[<C-\><C-N>]])  -- Cancel the :substitute command.
+    eq(0, eval("&modified"))
+  end)
+
   it('shows split window when typing the pattern', function()
     feed(":%s/tw")
     screen:expect([[
@@ -957,7 +981,7 @@ describe(":substitute, 'inccommand' with a failing expression", function()
     common_setup(screen, case, default_text)
   end
 
-  it('in the pattern does nothing for', function()
+  it('in the pattern does nothing', function()
     for _, case in pairs(cases) do
       refresh(case)
       execute("set inccommand=" .. case)