The fusion5 Keyboard add on is rather flexible like many tablet keyboards. When pressing the mouse it can leave to it pressing the plastic top in slightly which shifts your mouse position from where you actually wanted to go. The fix however is fairly simple, cheap and light. I am not specifying sizes below as sizes will change pending your own keyboard but the method of stiffening the board is universal.
First you'll need to peel the keyboard from the fabric case, nearly all I've seen always use double sided tape to bond the keyboard to the fabric case. So just push a finger between the keyboard and fabric and run it along the edges until it comes free.
BE CAREFUL with the ribbon cable (the orange cable).
Measure the bottom of the keyboard and cut some 3mm acrylic sheet to size. You will also need a slot where the ribbon cable can pass through.
The ribbon cables are held in place with the clipped plug, you will need to carefully pull the clip edges back to release the cable which you can then pull out, note the direction of the cable as you need to put this back!
Pass the cable through the perspex and reattach the ribbon cable remembering to push the catch back into place. Now you will need to apply double sided tape to the acrylic to stick back in place. I went a bit over the top but rather that then it fall off!
Re-positioning the keyboard is something you will want to take your time with, so do a mock run first before you peel off the opposite side of the double sided tape.
Keyboard back in place with my Fusion5 cradled. Obviously I've added 3mm to the total thickness but the case still closes fine. The sheet of acrylic weighs very little so there is no noticeable weight gain. However now the keyboard is braced, typing is much smoother and the mouse actually goes where it should lol.
Ready to rock and roll again!