Registering StyleHook for custom class solves issue:
TMyMemo = class(TMemo)
strict private
class constructor Create;
class destructor Destroy;
end;
class constructor TMyMemo.Create;
begin
TCustomStyleEngine.RegisterStyleHook(TMyMemo, TMemoStyleHook);
end;
class destructor TMyMemo.Destroy;
begin
TCustomStyleEngine.UnRegisterStyleHook(TMyMemo, TMemoStyleHook);
end;
There is bug in TStyleEngine.HandleMessage function, specifically in part that tries to find appropriate StyleHook class to handle messages
if RegisteredStyleHooks.ContainsKey(Control.ClassType) then
// The easy way: The class is registered
LStyleHook := CreateStyleHook(RegisteredStyleHooks[Control.ClassType])
else
begin
// The hard way: An ancestor is registered
for LItem in RegisteredStyleHooks do
if Control.InheritsFrom(LItem.Key) then
begin
LStyleHook := CreateStyleHook(Litem.Value);
Break;
end;
If StyleHook is registered for exact class then there is no problem, and appropriate StyleHook class will be returned. However, "the hard way" part is flawed. It will try to find class ancestor that has registered StyleHook. But it will return first ancestor it comes across. If it finds TEditStyleHook first (that is registered for TCustomEdit class), it will use that one instead of TMemoStyleHook. Since TEditStyleHook does not know how to handle scrollbars issue appears.
Randomness in buggy behavior is due the way RegisteredStyleHooks are stored. They are stored in dictionary where key is TClass. And order is determined by TClass hash that is basically pointer to class info and can change as you change code.
Issue is reported as RSP-10066 and there is attached project that reproduces it.
With help of following code it is easy to see how order of registered classes changes as you add/remove code and/or other controls.
type
TStyleHelper = class(TCustomStyleEngine)
public
class function GetClasses: TArray<TClass>;
end;
class function TStyleHelper.GetClasses: TArray<TClass>;
begin
Result := Self.RegisteredStyleHooks.Keys.ToArray;
end;
procedure TForm1.Button1Click(Sender: TObject);
var
LItem: TClass;
Classes: TArray<TClass>;
begin
Classes := TStyleHelper.GetClasses;
for LItem in Classes do
MyMemo1.Lines.Add(LItem.ClassName);
end;
