[SOLVED] TCheckListBox: Scroll first selected item to top

[Hansaplast]

I'm using a TCheckListBox with quite a few items in it (200+).
When at least one item has been checked, I'd like the list to scroll that item to the top (only at the form.onShow event - not while checking items).

No matter what I try, I seem to be unable to accomplish this.

What I have tried:

Code: Pascal  [Select][+][-]

1. TCheckListBox.TopIndex := X;

This only works when the checked item is not yet visible.

Code: Pascal  [Select][+][-]

1. TCheckListBox.ItemIndex := X;
2. TCheckListBox.MakeCurrentVisible;

This only scrolls the selected item into the visible area, as expeced, which often results the checked row to appear at the bottom of the visible area.

Code: Pascal  [Select][+][-]

1. TCheckListBox.ScrollBy(0, Y); // Y being an arbitrary number

This doesn't do anything at all, Y being positive or negative ... nothing happens.

Code: Pascal  [Select][+][-]

1. TCheckListBox.ScrollBy)WS(0, Y); // Y being an arbitrary number

I'm not sure what the purpose of this is, but the error "TWinControl.ScrollBy_WS: Handle not allocated" indicates that I cannot use this with Cocoa [see: documentation]).

I have no access to the vertical scrollbar with TChecListBox.

Does anyone know how this can be accomplished?
Or is this a bug as I would assume setting "TopIndex" would do what I'm looking for (documentation says: Index of the first visible (or top-most) item)?


For reference:
Lazarus 4.99 (rev main_4_99-3421-g449b0a887c) FPC 3.3.1 x86_64-darwin-cocoa, macOS Tahoe 26.4.1 using the latest Commandline Tools.

[Hansaplast]

Hacky work around:
Most certainly not the cleanest way and it feels kind-a sketchy way to do it this way, but for now for Cocoa this seems to work:

Checklistbox.TopIndex := Checklistbox.Count-1;

Code: Pascal  [Select][+][-]

1. for counter:=0 to Checklistbox.Count-1 do
2.   if Checklistbox.Checked[Counter] then
3.     begin
4.       Checklistbox.TopIndex:=Counter;
5.       break;
6.     end;

And this only works with an onPaint event of one of the other controls on the form (or parent TPanel).
This does not work in an onShow event somehow.

Just posting it here in case someone is running into the same issue.

[rich2014]

it should be the issue in TCheckListBox.TopIndex.

commit a bug report please.

[Hansaplast]

Thanks Rich014, will do 😊

[Hansaplast]

Bugtracker: 42259

[mas steindorff]

I understand this is marked solved but I have a suggestion.
I too had a long list of checkboxes (around 60) to work with but went to a tStringGrid after trying to enhance the checkboxes during run time. the Grid comes with more group operators.
see https://wiki.freepascal.org/TStringGrid
"CheckBox Column" section
MAS

[Hansaplast]

Oh cool, interesting approach - thank you for sharing 😊
I will look into this one as a backup solution for sure.