Hi Thomas,
thank you for your answer.
Already done. Saw nothing.
Screen was in 1280 x 900 mode or so. I also wear my glasses, but I still saw nothing (shared folder works fine on WIndows).
The shared folder is meant to be in Root:Media
Cleared that VM.
Drag and drop does not works too (but Clipboard <–> works) for both Linux.
Fired Linux Mint 18.1 (64Bits ! Xojo 2015r1 applications).
I do not see the shared folder either
Tried Firefox: works.
Upload my files (Application and a bunch of files to deal with that application), from the macOS host,
Download the zips, csv, whatever.
Found a bunch of bugs (specific to Linux: works fine on macOS / Windows *)
Squashed all bugs found excepted one:
I use Keyboard.AsyncOptionKey: nothing happens where it works on other OS.
I added Or Keyboard.ControlOptionKey: nothing happens too.
Used code:
If Keyboard.AsyncOptionKey Or Keyboard.AsyncAltKey Then
A bit boring to upload/download the files for testing.
Worst: I have to re-download if I quit VirtualBox. Installing Mint ask more than 10GB
(it seems that I used Mint as Live CD
)
At last: Mint is slow as hell. Click twice to launch the application, smoke a cigarette and when done, drink a cup of coffe, then the application appears on screen !
Problems: I do not smoke and do not drik coffee at home / expensive at McDonalds (a bunch of coffees
) and produce strange results on me: irritability, trembling, etc. 
Back home, I will clear this Mint install and add a brand new one with 20MB.
My main HD (boot device) is a 250GB SSD.