This is something I notice too when working on the laptop (1366x768 is currently the most common laptop resolution) and means having to move left and right a fair bit. Not a major problem but slightly irritating if entering a number of lines (particularly when using a trackpad!)
A collapsable side menu could help but if I may suggest a couple of modifications to the line layout itself, they may be of greater benefit, saving the need to hide the sidebar:
Reduce the width of the Qty field. It currently can accommodate a number eight digits long which is probably excessive for the majority of use cases. I’d suggest making this wide enough to display four digits.
Similarly, reduce the width of the Unit Price field. This can accommodate a number of 11,2 digits - so just shy of a billion in whichever currency. Again, I’d be fairly confident in saying there is no need for a number this large (and even if there is, I only recommendend reducing the width of the field, not the technical ability of the software to store such a large number). I’d suggest a width to accommodate 6,2 (i.e., just shy of ten thousand).
The platform that I’m migrating away from uses a partially two row layout for each line, moving the Item drop-down selection above the next field which might be something to consider. In fact, I can’t see any need for the height of the Account drop-down which only needs to be one line of text high. If this was sized down, the Item drop-down (similarly one line of text high) could fit above it.
If the final suggestion was implemented, other adjustments would be needed to ensure clear separation between each line (a border with, say, 5px of padding and 5px of margin below each line ) and it would obviously mean more vertical space is required. I’d argue that vertical scrolling is preferable to horizontal though.
That’s just a few thoughts on the matter which are hopefully useful.