Dialog Toolkit - Checkboxes behaving like text boxes

My applet has checkboxes by which the user indicates they wish to perform a function or change a setting. The checkboxes are created with Dialog Toolkit v1.1.3.

On my M1 Mac running macOS 13.3.1(a), one checkbox in one dialog, however, behaves like small text boxes. The same dialog on my old Mac Mini running macOS 10.13.6 has three checkboxes which behave like text boxes. Oddly, earlier today another checkbox had that behaviour but at present it does not. The text box behaviour occurs when running the applet within Script Debugger and as an exported run-only applet.

The behaviour is found by moving the mouse pointer over the checkboxes. The pointer changes to an “I” bar. Double-clicking in the checkbox seems to select its empty content and looks like this:


I can’t find anything in my applet which would make a checkbox behave like that. The code is in this form:

set theCheckBoxSaveSettingsLabel to localized string "Save current settings" from table "[My Applet]"
set {utilities_theCheckbox_Save_Settings, theTop} to create checkbox theCheckBoxSaveSettingsLabel left inset accViewInset bottom (theTop + 5) max width 200

All of the calls to “create checkbox” are in that form.

It does seem odd that more checkboxes are affected when running in macOS 10.13. Is there something which can prevent that behaviour ?