I’m trying to play Toccata by Debussy and the piece is in the key of E. In the sheet music, the F has a double sharp next to it, so that means I would be playing a G sharp. When I looked at the synthesia, it was playing a natural G. I’m not sure if I’m wrong or the synthesia is but I highlighted the section in yellow.
Accidentals affect the basic note - if you like, the white key on the piano.
So, regardless of the key signature - which permanently changes certain notes (here F, C, G and D♯), the double sharp applies to a basic F note, making it Fx (F♯♯) or F double sharp.Taking it up two semitones, so it looks like a G note on piano.
What it doesn't do is take the F♯ notated in the key signature, and then double sharp that!
A sharp or flat means you raise or lower the note by one half step. Or, as semitone. When a note has a small cross-like sign next to it, like you have there, you do this twice by raising it a whole step, two semitones. F > F# is one, F# > G is another.
Interestingly there is another double sharp there, before the bar line. In order to cancel a double sharp there would need to be a single sharp there (you did once also need a natural sign but this is no longer necessary).