extmarks: fix crash due to invalid column values in inccommand preview

This used to use -1 and MAXCOL values. Make sure in range values are
used.

src/nvim/extmark.c

@@ -699,21 +699,30 @@ void bufhl_add_hl_pos_offset(buf_T *buf,
 
   // TODO(bfredl): if decoration had blocky mode, we could avoid this loop
   for (linenr_T lnum = pos_start.lnum; lnum <= pos_end.lnum; lnum ++) {
+    int end_off = 0;
     if (pos_start.lnum < lnum && lnum < pos_end.lnum) {
-      hl_start = offset-1;
+      // TODO(bfredl): This is quite ad-hoc, but the space between |num| and
-      hl_end = MAXCOL;
+      // text being highlighted is the indication of \n being part of the
+      // substituted text. But it would be more consistent to highlight
+      // a space _after_ the previous line instead (like highlight EOL list
+      // char)
+      hl_start = MAX(offset-1, 0);
+      end_off = 1;
+      hl_end = 0;
     } else if (lnum == pos_start.lnum && lnum < pos_end.lnum) {
       hl_start = pos_start.col + offset;
-      hl_end = MAXCOL;
+      end_off = 1;
+      hl_end = 0;
     } else if (pos_start.lnum < lnum && lnum == pos_end.lnum) {
-      hl_start = offset-1;
+      hl_start = MAX(offset-1, 0);
       hl_end = pos_end.col + offset;
     } else if (pos_start.lnum == lnum && pos_end.lnum == lnum) {
       hl_start = pos_start.col + offset;
       hl_end = pos_end.col + offset;
     }
     (void)extmark_add_decoration(buf, (uint64_t)src_id, hl_id,
-                                 (int)lnum-1, hl_start, (int)lnum-1, hl_end,
+                                 (int)lnum-1, hl_start,
+                                 (int)lnum-1+end_off, hl_end,
                                  VIRTTEXT_EMPTY);
   }
 }

test/functional/ui/inccommand_spec.lua

@@ -18,6 +18,7 @@ local wait = helpers.wait
 local nvim = helpers.nvim
 local sleep = helpers.sleep
 local nvim_dir = helpers.nvim_dir
+local assert_alive = helpers.assert_alive
 
 local default_text = [[
   Inc substitution on
@@ -84,6 +85,7 @@ local function common_setup(screen, inccommand, text)
       [14] = {foreground = Screen.colors.White, background = Screen.colors.Red},
       [15] = {bold=true, foreground=Screen.colors.Blue},
       [16] = {background=Screen.colors.Grey90},  -- cursorline
+      [17] = {foreground = Screen.colors.Blue1},
       vis  = {background=Screen.colors.LightGrey}
     })
   end
@@ -2291,6 +2293,76 @@ describe(":substitute", function()
     ]])
   end)
 
+  it("inccommand=split, contraction of two subsequent NL chars", function()
+    -- luacheck: push ignore 611
+    local text = [[
+      AAA AA
+      
+      BBB BB
+      
+      CCC CC
+      
+]]
+    -- luacheck: pop
+
+    -- This used to crash, but more than 20 highlight entries are required
+    -- to reproduce it (so that the marktree has multiple nodes)
+    common_setup(screen, "split", string.rep(text,10))
+    feed(":%s/\\n\\n/<c-v><c-m>/g")
+    screen:expect{grid=[[
+      CCC CC                        |
+      AAA AA                        |
+      BBB BB                        |
+      CCC CC                        |
+                                    |
+      {11:[No Name] [+]                 }|
+      | 1| AAA AA                   |
+      | 2|{12: }BBB BB                   |
+      | 3|{12: }CCC CC                   |
+      | 4|{12: }AAA AA                   |
+      | 5|{12: }BBB BB                   |
+      | 6|{12: }CCC CC                   |
+      | 7|{12: }AAA AA                   |
+      {10:[Preview]                     }|
+      :%s/\n\n/{17:^M}/g^                 |
+    ]]}
+    assert_alive()
+  end)
+
+  it("inccommand=nosplit, contraction of two subsequent NL chars", function()
+    -- luacheck: push ignore 611
+    local text = [[
+      AAA AA
+      
+      BBB BB
+      
+      CCC CC
+      
+]]
+    -- luacheck: pop
+
+    common_setup(screen, "nosplit", string.rep(text,10))
+    feed(":%s/\\n\\n/<c-v><c-m>/g")
+    screen:expect{grid=[[
+      CCC CC                        |
+      AAA AA                        |
+      BBB BB                        |
+      CCC CC                        |
+      AAA AA                        |
+      BBB BB                        |
+      CCC CC                        |
+      AAA AA                        |
+      BBB BB                        |
+      CCC CC                        |
+      AAA AA                        |
+      BBB BB                        |
+      CCC CC                        |
+                                    |
+      :%s/\n\n/{17:^M}/g^                 |
+    ]]}
+    assert_alive()
+  end)
+
   it("inccommand=split, multibyte text", function()
     common_setup(screen, "split", multibyte_text)
     feed(":%s/£.*ѫ/X¥¥")