Why do hidden forms and controls stop painting correctly after switching themes twice?

Question

I hate the title of this question. Anyway:

If you call TForm.Show with a custom theme (Windows10 Dark in this case), then close that form, then change the theme to the system Windows theme, then change back to the Windows10 Dark theme, and finally call TForm.Show on that form again, the border renders incorrectly and certain controls do not render properly, like a TComboBox.

I have a test project below, and a "fix" of sorts. But I do not like my fix and the reason for this question is that I do not really understand what is happening here that causes the form to render incorrectly only if it was hidden while the theme changed, and only if the theme is changed away from, and then back to, Windows10 Dark.

My fix is to track the theme change. If the condition I describe above occurs, I intercept the CM_SHOWINGCHANGED message, ignore it, then force the window to be recreated and then process the inherited CM_SHOWINGCHANGED the next time around. It is a very brittle fix and obviously not the way to go, so I am hoping someone can show me what is actually happening so I can fix it "for real."

Incidentally, I have submitted this as a bug to Embarcadero already. https://quality.embarcadero.com/browse/RSP-33977

Here is the test code. You'll need to add Windows10 Dark to the application's styles, obviously.

unit Unit22;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, Vcl.ExtCtrls, Unit23, Vcl.Themes;

type
  TForm22 = class(TForm)
    Panel1: TPanel;
    ComboBox1: TComboBox;
    RadioGroup1: TRadioGroup;
    ButtonShow: TButton;
    Memo1: TMemo;
    procedure ButtonShowClick(Sender: TObject);
    procedure RadioGroup1Click(Sender: TObject);
    procedure FormShow(Sender: TObject);
  private
    { Private declarations }
    FAllowChange: Boolean;
  public
    { Public declarations }
  end;

var
  Form22: TForm22;

implementation

{$R *.dfm}

procedure TForm22.ButtonShowClick(Sender: TObject);
begin
  Form23.Show;
end;

procedure TForm22.FormShow(Sender: TObject);
begin
  if StyleServices.Name = 'Windows10 Dark' then
    RadioGroup1.ItemIndex := 1
  else
    RadioGroup1.ItemIndex := 0;
  FAllowChange := True;
end;

procedure TForm22.RadioGroup1Click(Sender: TObject);
begin
  if not FAllowChange then
    exit;
  if RadioGroup1.ItemIndex = 0 then
    TStyleManager.SetStyle('Windows');
  if RadioGroup1.ItemIndex = 1 then
    TStyleManager.SetStyle('Windows10 Dark');
end;

end.

Unit 22 DPR:

object Form22: TForm22
  Left = 0
  Top = 0
  ActiveControl = Memo1
  Caption = 'Form22'
  ClientHeight = 305
  ClientWidth = 511
  Color = clBtnFace
  Font.Charset = DEFAULT_CHARSET
  Font.Color = clWindowText
  Font.Height = -11
  Font.Name = 'Tahoma'
  Font.Style = []
  OldCreateOrder = False
  OnShow = FormShow
  PixelsPerInch = 96
  TextHeight = 13
  object Panel1: TPanel
    Left = 0
    Top = 0
    Width = 511
    Height = 305
    Align = alClient
    BevelEdges = []
    BevelOuter = bvNone
    Caption = 'Panel1'
    ShowCaption = False
    TabOrder = 0
    object ComboBox1: TComboBox
      Left = 16
      Top = 8
      Width = 145
      Height = 21
      Style = csDropDownList
      ItemIndex = 0
      TabOrder = 0
      Text = 'one'
      Items.Strings = (
        'one'
        'two'
        'three')
    end
    object RadioGroup1: TRadioGroup
      Left = 16
      Top = 48
      Width = 185
      Height = 105
      Caption = 'RadioGroup1'
      Items.Strings = (
        'windows'
        'dark')
      TabOrder = 1
      OnClick = RadioGroup1Click
    end
    object ButtonShow: TButton
      Left = 16
      Top = 159
      Width = 75
      Height = 25
      Caption = 'ButtonShow'
      TabOrder = 2
      OnClick = ButtonShowClick
    end
    object Memo1: TMemo
      Left = 207
      Top = 8
      Width = 274
      Height = 281
      Lines.Strings = (
        'Always start in dark.'
        ''
        'Steps to reproduce:'
        '1.'#9'Click ButtonShow.'
        '2.'#9'Close the window that opened.'
        '3.'#9'Click Windows (change to system them).'
        '4.'#9'Click Dark (change back to dark VCL style).'
        '5.'#9'Click ButtonShow again. The controls are '
        'not properly painted. Combobox text is black and form '
        'is wrong until resize.'
        ''
        'Hacky fix:'
        '1.'#9'Click ButtonShow.'
        '2.'#9'Check the '#8220'Fix'#8221' button in the window that '
        'opened, then close it.'
        '3.'#9'Click Windows (change to system)'
        '4.'#9'Click Dark (change back to vcl dark)'
        '5.'#9'Click ButtonShow. See comments in source.'
        '')
      ReadOnly = True
      TabOrder = 3
    end
  end
end

Unit23:

unit Unit23;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, Vcl.ExtCtrls, Vcl.Themes;

type
  TForm23 = class(TForm)
    Panel1: TPanel;
    ComboBox1: TComboBox;
    ComboBox2: TComboBox;
    ComboBox3: TComboBox;
    Button1: TButton;
    CheckBox1: TCheckBox;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
    FFixing: Boolean;
    FNeedFix: String;
    FShowedStyle: String;
  protected
    procedure DoShow; override;
  public
    { Public declarations }
    procedure CMStyleChanged(var Message: TMessage); message CM_STYLECHANGED;
    procedure CMShowingChanged(var Message: TMessage); message CM_SHOWINGCHANGED;
  end;

var
  Form23: TForm23;

implementation

{$R *.dfm}

procedure TForm23.Button1Click(Sender: TObject);
begin
  PostMessage(Handle, CM_RECREATEWND, 0, 0);
end;

procedure TForm23.CMShowingChanged(var Message: TMessage);
var
  DoFix: Boolean;
begin
  if not Showing then
    inherited
  else
  begin
    // if the theme changed away from dark, then back to dark, while we were
    // not visible, then we need to force the window to be recreated again
    // before showing.

    // This is a really bad hack but basically I am just preventing the
    // normal response to CMShowingChanged and then setting up a message
    // queue that will recreate the window and then process the CM_SHOWINGCHANGED
    // message again. This will probably break the universe but it appears to work
    // in this test.

    FShowedStyle := StyleServices.Name;
    Panel1.Caption := FShowedStyle;
    DoFix := not FFixing and (FNeedFix <> '') and (FNeedFix = FShowedStyle);
    FNeedFix := '';

    if DoFix and CheckBox1.Checked then
    begin
      FFixing := True;
      // SendMessage(Handle, WM_SETREDRAW, Winapi.Windows.WPARAM(LongBool(False)), 0);
      PostMessage(Handle, CM_RECREATEWND, 0, 0);
      // PostMessage(Handle, CM_SHOWINGCHANGED, Message.WParam, Message.LParam);
      // do not allow inherited.
    end else
    begin
      FFixing := False;
      inherited;
    end;
  end;
end;

procedure TForm23.CMStyleChanged(var Message: TMessage);
begin
  FNeedFix := FShowedStyle;
  inherited;
end;

procedure TForm23.DoShow;
var
  DoFix: Boolean;
begin
  inherited;
  exit;
end;

end.

Unit23 DPR:

object Form23: TForm23
  Left = 0
  Top = 0
  Caption = 'Form23'
  ClientHeight = 253
  ClientWidth = 360
  Color = clBtnFace
  Font.Charset = DEFAULT_CHARSET
  Font.Color = clWindowText
  Font.Height = -11
  Font.Name = 'Tahoma'
  Font.Style = []
  OldCreateOrder = False
  PixelsPerInch = 96
  TextHeight = 13
  object Panel1: TPanel
    Left = 0
    Top = 0
    Width = 360
    Height = 253
    Align = alClient
    Alignment = taRightJustify
    BevelEdges = []
    BevelOuter = bvNone
    Caption = 'Panel1'
    TabOrder = 0
    object ComboBox1: TComboBox
      Left = 16
      Top = 32
      Width = 145
      Height = 21
      Style = csDropDownList
      ItemIndex = 0
      TabOrder = 0
      Text = 'one'
      Items.Strings = (
        'one'
        'two'
        'three')
    end
    object ComboBox2: TComboBox
      Left = 16
      Top = 59
      Width = 145
      Height = 21
      Style = csDropDownList
      ItemIndex = 1
      TabOrder = 1
      Text = 'two'
      Items.Strings = (
        'one'
        'two'
        'three')
    end
    object ComboBox3: TComboBox
      Left = 16
      Top = 86
      Width = 145
      Height = 21
      Style = csDropDownList
      ItemIndex = 2
      TabOrder = 2
      Text = 'three'
      Items.Strings = (
        'one'
        'two'
        'three')
    end
    object Button1: TButton
      Left = 16
      Top = 136
      Width = 75
      Height = 25
      Caption = 'RecreateWnd'
      TabOrder = 3
      OnClick = Button1Click
    end
    object CheckBox1: TCheckBox
      Left = 16
      Top = 167
      Width = 273
      Height = 17
      Caption = 'Fix with CM_SHOWINGCHANGED hack'
      TabOrder = 4
    end
  end
end