In 'Set/Replace' mode this is not a problem, but 'Extend' or 'Subtract'
modes were useless with the current behavior.
The problem here is that 'node.select' fires before 'node.select_box'
(which is fine) but deselects immediately on click.
This issue has come up before in other editors, see
T70457: Dopesheet click-drag-selection deselects channels [e.g. breaking copy-pasting keys workflow]
rB395dfff103e1: Dopesheet: Avoid deselect-all triggering on every box-select
rBa8ea1ea1b7d5: Fix: Broken tweak select-transform in Dopesheet
Now delay deselection in empty space to mouse release (same as done in
before mentioned report).