How to create a dialog like component that allows drop other controls inside it?

Take a closer look at TTabControl / TTabItem in the unit FMX.TabControl. This is your perfect example because it basically needs to solve the same problem.

The following function is what you need to override:

procedure DoAddObject(const AObject: TFmxObject); override;

This is called when a control is added to your control. Override this function so that your control is added to the FpnlClientArea control instead. You’d get something similar to this:

procedure TNaharFMXPopup.DoAddObject(const AObject: TFmxObject);
// ...
begin
  if (FpnlClientArea <> nil) and not AObject.Equals(FpnlClientArea) and not AObject.Equals(ResourceLink) then
  begin
    FpnlClientArea.AddObject(AObject);
  end
  else
    inherited;
end;

Make sure that AObject.Equals also excludes your other “not stored” controls.

Without the DoAddObject override, the FMX TabControl would show the same problem as your component currently has.

The TPopup is not intended to accept controls. So that needs a few more tricks.
Here’s a modified version of your unit that works for me. I’ve added a few comments:

unit NaharFMXPopup;

interface

uses
  System.UITypes,
  System.Variants,
  System.SysUtils, System.Classes, FMX.Types, FMX.Controls, FMX.Layouts, FMX.StdCtrls;

type
  [ComponentPlatformsAttribute(pidWin32 or pidWin64 or pidOSX32 or pidiOSSimulator or pidiOSDevice or pidAndroid)]
  TNaharFMXPopup = class(TPopup)
  private
    procedure   ApplyControlsProp;
  protected
    FpnlMain       : TPanel;
    FlytToolBar    : TLayout;
    FbtnClose      : TButton;
    FbtnSave       : TButton;
    FbtnEdit       : TButton;
    FpnlClientArea : TContent; // change to TContent. 
    // For TPanel we'd have to call SetAcceptControls(False), 
    // but that is not easily possible because that is protected
    FlblTitle      : TLabel;
    procedure   Loaded; override;
    procedure   Notification(AComponent: TComponent; Operation: TOperation); override;
    procedure   DoAddObject(const AObject: TFmxObject); override;
  public
    procedure   InternalOnClose(Sender: TObject);
    procedure   InternalOnSave(Sender: TObject);
    procedure   InternalOnEdit(Sender: TObject);
    constructor Create(AOwner: TComponent); override;
    destructor  Destroy; override;
    procedure   SetEvents;
  published
  end;

implementation


{ TNaharFMXPopup }

constructor TNaharFMXPopup.Create(AOwner: TComponent);
begin
  inherited;

  FpnlMain         := TPanel.Create(Self);
  FlblTitle        := TLabel.Create(Self);
  FlytToolBar      := TLayout.Create(Self);
  FbtnEdit         := TButton.Create(Self);
  FpnlClientArea   := TContent.Create(Self); // change to TContent
  FbtnClose         := TButton.Create(FlytToolBar);
  FbtnSave          := TButton.Create(FlytToolBar);

  Height         := 382;
  Placement      := TPlacement.Center;
  StyleLookup    := 'combopopupstyle';
  Width          := 300;

  // A TPopup is not intended to accept controls
  // so we have to undo those restrictions:
  Visible := True;
  SetAcceptsControls(True);

  ApplyControlsProp;
end;

destructor TNaharFMXPopup.Destroy;
begin

  inherited;
end;

procedure TNaharFMXPopup.ApplyControlsProp;
begin
  with FpnlMain do
  begin
    Parent         := Self;
    Align          := TAlignLayout.Bottom;
    StyleLookup    := 'grouppanel';
    TabOrder       := 0;
    Height         := 50;
    Margins.Bottom := 10;
    Margins.Left   := 10;
    Margins.Right  := 10;
    Margins.Top    := 10;
    Stored         := false;
  end;
  with FpnlClientArea do
  begin
    Parent         := Self; // we have to change this to Self (it refuses working if the parent is FPnlMain)
    Align          := TAlignLayout.Client;
    Margins.Left   := 3;
    Margins.Right  := 3;
    Margins.Top    := 3;
    Margins.Bottom := 3;
    Stored         := false;
  end;
  with FlytToolBar do
  begin
    Parent         := FpnlMain;
    Align          := TAlignLayout.Bottom;
    Height         := 50;
    Stored         := false;
  end;
  with FbtnClose do
  begin
    Parent         := FlytToolBar;
    Text           := 'Close';
    Align          := TAlignLayout.Left;
    Height         := 50;
    StyleLookup    := 'tilebutton';
    TabOrder       := 0;
    Width          := 70;
    ModalResult    := mrClose;
    Stored         := false;
  end;
  with FbtnEdit do
  begin
    Parent         := FlytToolBar;
    Text           := '';//'Edita';
    Align          := TAlignLayout.Left;
    Height         := 50;
    StyleLookup    := 'tilebutton';
    TabOrder       := 1;
    Width          := 70;
    ModalResult    := mrContinue;
    Stored         := false;
    Enabled        := false;
  end;
  with FbtnSave do
  begin
    Parent         := FlytToolBar;
    Text           := 'Save';
    Align          := TAlignLayout.Left;
    Height         := 50;
    StyleLookup    := 'tilebutton';
    TabOrder       := 2;
    Width          := 70;
    ModalResult    := mrOk;
    Stored         := false;
  end;
end;

procedure TNaharFMXPopup.Loaded;
begin
  inherited;

  ApplyControlsProp;
//  SetEvents;

end;

procedure TNaharFMXPopup.Notification(AComponent: TComponent;
  Operation: TOperation);
begin
  inherited;

end;

procedure TNaharFMXPopup.InternalOnClose(Sender: TObject);
begin
end;

procedure TNaharFMXPopup.InternalOnEdit(Sender: TObject);
begin
end;

procedure TNaharFMXPopup.InternalOnSave(Sender: TObject);
begin
end;

procedure TNaharFMXPopup.SetEvents;
begin
  FbtnClose.OnClick := InternalOnClose;
  FbtnSave.OnClick := InternalOnSave;
  FbtnEdit.OnClick := InternalOnEdit;
end;


procedure TNaharFMXPopup.DoAddObject(const AObject: TFmxObject);
begin
//inherited; try commenting the block bellow and uncommenting this one
//Exit;

  if (FpnlClientArea <> nil)
    and not AObject.Equals(FpnlClientArea)
    and not AObject.Equals(ResourceLink)
    and not AObject.Equals(FpnlMain)
    and not AObject.Equals(FlblTitle)
    and not AObject.Equals(FlytToolBar)
    and not AObject.Equals(FbtnEdit)
    and not AObject.Equals(FpnlClientArea)
    and not AObject.Equals(FbtnClose)
    and not AObject.Equals(FbtnSave) then

  begin
    FpnlClientArea.AddObject(AObject);
  end
  else
    inherited;
end;

end.