Drag-and-drop element to selection behaves differently than double-clicking
There is an inconsistency relating to how elements are applied to notes in a selection. When double-clicking on elements (articulations, for instance), they are applied to all notes and chords in a selection, while lines are spanned for the selection duration. But if the user click and drags the articulation or lines from the palettes into the selection, they are applied differently: articulations are applied only to the single note you above which you released it (or, if you released inside the selection but not above a notehead, no note gets an articulation). Regarding the lines, here are some concrete examples of the inconsistencies of behaviour observe:
* select multiple notes and press 's' and a slur will cover all these notes. But select multiple notes and drag a slur from the lines palette and it applies to a single note and the one immediately after it.
* hairpins work a bit better though: dragging and releasing a hairpin above a note in a selection will extend it until the end of the bar. But using the shortcut '<' or '>' will add hairpins below the whole selection as expected.
* likewise, using articulations shortcuts with a selection (such as 'Shift+S) adds articulations to all notes.
My suggestion regarding this would be: first, apply the same behavior to drag and dropping as to the current keyboard shortcuts. Secondly, when a selection is made, drag and dropping anywhere inside the blue rectangle should apply the element to all notes instead of requiring a precise drop on a small notehead.
Discussion link: https://musescore.org/en/node/289996#comment-923394