Problems with Rectange Select Tool (Edit mode) in the Song Editor

Nobody has claimed this yet.

1. If the view is scrolled while performing a rectangle-drag select, the rectangle's starting point doesn't "stick" to the sequencer's content (background), instead it scrolls with the view (foreground) leaving previously selected items unselected in the end.
2. There's no way to add or substract from the selection once it's done - the only thing possible is replacing the current selection. This makes selecting non-rectangular areas impossible.
3. I'd also suggest making it possible to select/deselect items with Ctrl + Left Mouse Button (like in a typical file manager) I see shift is taken for something but a way to select clips from A to B might also be useful.

Start in the Song Editor's rectangle-selection handling and reproduce the scrolling case, current replacement-only behavior, and modifier-click requests. Done means the reported selection operations work consistently, including adding or removing items and preserving the intended selection while scrolling.

Written by the indexing model from the issue text.