When I scroll via right arrow key and there is a chord with 3 notes for instance then only two of them get played and selected when I scroll with the right arrow key before Dorico jumps to the next note.
Even more strange: When I set a slur from one note to the next quarter note of the following bar which ist in the next system (no special system break but set from Dorico itself) then scrolling into the next system gets impossible via right arrow key.
This seems to be a bug.
Here is a file to test.
scrolling anomalies.dorico.zip (206 KB)
It is to be expected that only two notes of the chord in bar 10 of your example project get selected: the navigation in Dorico is graphical in nature, so when you arrive on the D4 note and hit right arrow, the E4 note is selected not specifically because it is in the same voice or in the same chord, but simply because it is graphically to the right of the D4 note. This is certainly a different approach to what you might be used to from other scoring software, but it has the distinct advantage that you can navigate to any item of any type simply with the arrow keys, without needing to learn difficult-to-remember orders of selection when using Tab and Shift-Tab, and so on.
Dorico getting hung up on the slur at the end of the first system is very similar to the problem you reported previously, which we have somewhat improved for Dorico 1.1, but not this specific case. You can use down arrow to navigate to the system below, then Ctrl+left arrow to navigate back to the start of the system.