Don't allow Application.Restore to show WizardForm if it's hidden.

Projects/Src/Setup.WizardForm.pas

@@ -211,6 +211,7 @@ type
     procedure UpdatePage(const PageID: Integer);
     procedure UpdateSelectTasksPage;
     procedure WMSysCommand(var Message: TWMSysCommand); message WM_SYSCOMMAND;
+    procedure WMWindowPosChanging(var Message: TWMWindowPosChanging); message WM_WINDOWPOSCHANGING;
   public
     { Public declarations }
     PrepareToInstallFailureMessage: String;
@@ -2684,6 +2685,22 @@ begin
     inherited;
 end;
 
+procedure TWizardForm.WMWindowPosChanging(var Message: TWMWindowPosChanging);
+begin
+  { Work around a VCL issue (Delphi 11.3) when MainFormOnTaskBar=True:
+    If Application.Restore is called while the main form is hidden
+    (Visible=False), the window can become visible because of the SW_RESTORE
+    command it uses, which both unminimizes and shows a window. Reproducer:
+      Application.Minimize;
+      Hide;
+      Application.Restore;
+    This blocks any attempt to show the window while Visible=False.
+    (SW_RESTORE will still unminimize the window; it just cannot show it.) }
+  inherited;
+  if not Visible then
+    Message.WindowPos.flags := Message.WindowPos.flags and not SWP_SHOWWINDOW;
+end;
+
 procedure TWizardForm.LicenseAcceptedRadioClick(Sender: TObject);
 begin
   if CurPageID = wpLicense then