Have to say this doesn’t give a good first impression of Xojo (the labels & text boxes are all out of alignment) :
(er … how do I upload an image to a forum post?
Upload the image into a service and take the forum link, then in this forum, click in the Image icon (above the text at message writing) and Paste the forum link into it.
It might be the version of GTK you’re running on. GTK3 will need some additional help. More info in this thread:
https://forum.xojo.com/48126-gtk3-theming-modgtk3-resolves-layout-corruption-under-all-linux/last
Have you looked into that thread? Adding the modgtk3 module to your app will sort the 95th percentile of issues for you with no further work.
I use DropBox to share images. When you paste the link, change the last bit to “dl=0” to “raw=1” when you add the Image link so that it is displayed in the post:
https://www.dropbox.com/s/j7nfyo1bbhtnko3/SamplePost.png?dl=0becomes:
https://www.dropbox.com/s/j7nfyo1bbhtnko3/SamplePost.png?raw=1