msg: do not scroll entire screen (#8088)

src/nvim/highlight_defs.h

@@ -87,6 +87,7 @@ typedef enum {
   , HLF_QFL         // selected quickfix line
   , HLF_0           // Whitespace
   , HLF_INACTIVE    // NormalNC: Normal text in non-current windows
+  , HLF_MSGSEP      // message separator line
   , HLF_COUNT       // MUST be the last one
 } hlf_T;
 
@@ -137,7 +138,8 @@ EXTERN const char *hlf_names[] INIT(= {
   [HLF_MC] = "ColorColumn",
   [HLF_QFL] = "QuickFixLine",
   [HLF_0] = "Whitespace",
-  [HLF_INACTIVE] = "NormalNC"
+  [HLF_INACTIVE] = "NormalNC",
+  [HLF_MSGSEP] = "MsgSeparator",
 });