Hi everyone,
This is being posted so that anyone in the future can search Xojo with the tagNMHDR structure and will find this solution.
I created an Up-Down API Control in Windows 10 and was able to have the structure work in 32-bit mode, and there were output issues in 64-bit mode. Julian pointed to a similar structure alignment issue in <https://xojo.com/issue/51124>.
The workaround has the following two structures, with one for 32-bit applications, and the other for 64-bit applications:
32-bit applications (change gear in structure to include in only 32-bit applications)
Structure tagNMHDR
hWndForm as Integer
idForm as UInteger
code as UInt32
End Structure
64-bit applications (change gear in structure to include in only 64-bit applications)
Structure tagNMHDR
hWndForm as Integer
idForm as UInteger
code as UInt32
_padding As UInt32
End Structure
Thanks for your help Julian, much appreciated!