Prevent :topleft, etc modifying the inccommand preview window

src/nvim/ex_cmds.c

@@ -5535,6 +5535,9 @@ static buf_T *show_sub(exarg_T *eap, pos_T old_cusr,
 
   // We keep a special-purpose buffer around, but don't assume it exists.
   buf_T *preview_buf = bufnr ? buflist_findnr(bufnr) : 0;
+  // disable :leftabove/botright modifiers
+  // (especially ones that conflict with our win_split() call below)
+  cmdmod.split = 0;
   cmdmod.tab = 0;                 // disable :tab modifier
   cmdmod.noswapfile = true;       // disable swap for preview buffer
   // disable file info message

test/functional/ui/inccommand_spec.lua

@@ -95,7 +95,7 @@ local function common_setup(screen, inccommand, text)
   end
 end
 
-describe(":substitute, inccommand=split", function()
+describe(":substitute, inccommand=split interactivity", function()
   before_each(function()
     clear()
     common_setup(nil, "split", default_text)
@@ -788,6 +788,52 @@ describe(":substitute, inccommand=split", function()
     screen:expect{any=[[two lines]]}
   end)
 
+  it("ignores new-window modifiers when splitting the preview window", function()
+    -- one modifier
+    feed(':topleft %s/tw/to')
+    screen:expect([[
+      Inc substitution on           |
+      {12:to}o lines                     |
+      Inc substitution on           |
+      {12:to}o lines                     |
+                                    |
+      {11:[No Name] [+]                 }|
+      |2| {12:to}o lines                 |
+      |4| {12:to}o lines                 |
+      {15:~                             }|
+      {15:~                             }|
+      {15:~                             }|
+      {15:~                             }|
+      {15:~                             }|
+      {10:[Preview]                     }|
+      :topleft %s/tw/to^             |
+    ]])
+    feed('<Esc>')
+    screen:expect{any=[[two lines]]}
+
+    -- multiple modifiers
+    feed(':topleft vert %s/tw/to')
+    screen:expect([[
+      Inc substitution on           |
+      {12:to}o lines                     |
+      Inc substitution on           |
+      {12:to}o lines                     |
+                                    |
+      {11:[No Name] [+]                 }|
+      |2| {12:to}o lines                 |
+      |4| {12:to}o lines                 |
+      {15:~                             }|
+      {15:~                             }|
+      {15:~                             }|
+      {15:~                             }|
+      {15:~                             }|
+      {10:[Preview]                     }|
+      :topleft vert %s/tw/to^        |
+    ]])
+    feed('<Esc>')
+    screen:expect{any=[[two lines]]}
+  end)
+
   it('shows split window when typing the pattern', function()
     feed(":%s/tw")
     screen:expect([[