inccommand: Preview :sub commands only after the delimiter is present

Closes #5888

src/nvim/ex_docmd.c

@@ -9674,9 +9674,20 @@ bool cmd_can_preview(char_u *cmd)
   if (*ea.cmd == '*') {
     ea.cmd = skipwhite(ea.cmd + 1);
   }
-  find_command(&ea, NULL);
+  char_u *end = find_command(&ea, NULL);
 
-  return ea.cmdidx == CMD_substitute
+  switch (ea.cmdidx) {
-      || ea.cmdidx == CMD_smagic
+    case CMD_substitute:
-      || ea.cmdidx == CMD_snomagic;
+    case CMD_smagic:
+    case CMD_snomagic:
+      // Only preview once the pattern delimiter has been typed
+      if (*end && !ASCII_ISALNUM(*end)) {
+        return true;
+      }
+      break;
+    default:
+      break;
+  }
+
+  return false;
 }

test/functional/ui/inccommand_spec.lua

@@ -42,6 +42,7 @@ 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
+      vis  = {background=Screen.colors.LightGrey}
     })
   end
 
@@ -207,6 +208,42 @@ describe(":substitute, 'inccommand' preserves", function()
     end)
   end
 
+  for _, case in pairs{"", "split", "nosplit"} do
+    it("visual selection for non-previewable command (inccommand="..case..") #5888", function()
+      local screen = Screen.new(30,10)
+      common_setup(screen, case, default_text)
+      feed('1G2V')
+
+      feed(':s')
+      screen:expect([[
+        {vis:Inc substitution on}           |
+        t{vis:wo lines}                     |
+                                      |
+        {15:~                             }|
+        {15:~                             }|
+        {15:~                             }|
+        {15:~                             }|
+        {15:~                             }|
+        {15:~                             }|
+        :'<,'>s^                       |
+      ]])
+
+      feed('o')
+      screen:expect([[
+        {vis:Inc substitution on}           |
+        t{vis:wo lines}                     |
+                                      |
+        {15:~                             }|
+        {15:~                             }|
+        {15:~                             }|
+        {15:~                             }|
+        {15:~                             }|
+        {15:~                             }|
+        :'<,'>so^                      |
+      ]])
+    end)
+  end
+
 end)
 
 describe(":substitute, 'inccommand' preserves undo", function()