Add comment. Hope it makes some sense.

Components/ScintEdit.pas

@@ -1825,8 +1825,22 @@ procedure TScintEdit.StyleNeeded(const EndPos: Integer);
       var Section := TInnoSetupStyler.GetSectionFromLineState(FStyler.LineState);
       if Section <> scNone then
         Call(SCI_SETFOLDLEVEL, I, SC_FOLDLEVELBASE+1)
-      else
+      else begin
+        { Everything outside a section should have the header flag, even if it's
+          just a blank line or a comment. Doing this doesn't cause many fold
+          markers: if two lines have the same level and header flag the first
+          line doesn't get a fold mark since there's nothing to expand.
+          Not doing this however is a problem: for example, if the first
+          line is empty and and the second line starts a section then those two
+          lines would logically considered to be in the same 'fold' and edits
+          on the first line would affect the section if collapsed.
+          Did notice an issue (Scintilla bug?): Add a section with some lines.
+          Collapse it. Break the section header for example by removing ']'.
+          Scintialla now auto expands the section and removes the fold mark.
+          Retype the ']'. Scintilla now displays the old fold mark to expand the
+          section but it's already expanded.  }
         Call(SCI_SETFOLDLEVEL, I, SC_FOLDLEVELBASE or SC_FOLDLEVELHEADERFLAG);
+      end;
     end;
 
     Result := LastLine;