Should be able to "apply right"/"apply left" from a selection with non-change lines
If your cursor is on a line of change (red, green or blue), you can press Alt+arrow to apply that change to the file on the left or on the right.
However, if you select some text which contains a change, but also non-change text; or several changes and non-change text - Alt+arrow won't work, despite the semantics being very straightforward: Apply all overlapped changes, just as though you would have applied them by press Alt+arrow in each line the selection overlaps.
Using Meld 3.20 on Devuan GNU/Linux 3.