Michel, off topic here, but for web apps, those long presses could be handled better by hacking in touch event handlers on the client side. Or a feature request add-on to this bug:
<https://xojo.com/issue/23681>
My problem seems more fundamental. The trackpad literally isn’t generating right clicks. Plug a two-button mouse in, I can right click just fine. Might be an issue with the touchscreen feature of this Chromebook.
Sure, the client side could probably better manage it. If the Web app can gain access to it. From what I could figure, Xojo does not seem able to do it. Until the feature is added, the idea is to find a way to add long tap to an app through a few lines of Xojo code. I offered my solution, but any idea and better ways are welcome 
I do not know Java. Maybe there is a solution in that language that a knowledgeable member can share ?
For desktop apps, I do not have a touchscreen device to experiment with, and it could be very well that the system already manages to interpret long tap as right click, so Xojo apps be fine. But if that was not the case, measuring the time the user holds his finger between MouseDown and MouseUp seems like a pragmatic workaround.
On Windows laptops, the trackpad has two buttons to left and right click. On Chromebook, it seems you just click with two fingers.
I found that tip at Use your Chromebook touchpad - Chromebook Help
On Windows 8(.1) tablets and touch screen laptops the “long tap” works indeed for desktop apps as a right click.
Another option is to install GalliumOS with chrx
Instructions on GalliumOS website - youll still need to modify the bios but I found the whole installation process on my ASUS Apollo Lake chromebook much more straightforward, can dual boot and XOJO works really quite well in GalliumOS…
Edit: worth mentioning GalliumOS designed specifically for chromebooks all the function keys work perfectly etc