tl;dr: Selecting is complicated.
I have observed selecting behavior carefully since Macs first got cursor keys in the late ’80s. The difference is in the concept of anchoring. The older, more basic method is to always extend in the direction of the arrow key. The old Macintosh Finder worked this way when selecting items.
The more advanced method (first promulgated by Microsoft Word, I believe) is to treat the origin of the selection as an anchor point and extend or contract the selection relative to it. Most of us are used to this by now and this is what you’re expecting in Dorico. But making this work intuitively and usefully requires much more complex behavior.
Examples: You can Shift-Option-left & right arrow to extend the selection by whole words – as defined by spaces and certain other characters; these are custom definitions that differ from one program to another. Does the selection include or omit the trailing space? That differs per program too. You can double-click a word and Shift-click anywhere within another word (or double-click and drag) to extend the selection by whole words, because the program remembers that you started selecting with a double-click. This works in Word, TextEdit, Safari, etc., but not in BBEdit, in which I’m composing this post.
More in Safari
In Safari 16 (in which I’m editing this) it’s even subtler: When I Shift-click before a whole-word selection, Safari adds the space after the last word as well! (Good for dragging to move a selection.) If I do it again, the next whole word is also added! (Not so good.)
Another example: Select a range of text, and Shift-click outside that to add more to the selection. Then Shift-left or right arrow: the anchor is determined by which way you select with the keyboard! The first keypress always extends.
This is all great with text. Even with multiple lines, selecting up & down is still all in one data stream. But music scores need to be selectable in two dimensions, and I think the amount of complexity involved there for the ‘anchor’ method of selecting would be too much for both programmers and users to sort out.