I'm not at my Reaper PC at the moment, but I know I have mapped actions like this to shortcut keys.
Try searching for action with "edge" in the name (and perhaps also "item" or "midi").
I have mapped actions to move the left edge of the item to the left/right, and to move the right edge of the item to the left/right, so that I can lengthen/shorten either edge of the item. There are separate actions that move the edges by grid size and by pixel, so try adding "grid" or "pixel" to your search. Hopefully you'll find them on your own.
If you haven't already come up with a solution by the time I get home (15 hours), I'll post back with the actions I use.
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Best Regards, Ernie "lunker" Lundqvist
BDSM (Bad Dog Studio Musicians)
Windows 10 running on Z390 + i7-8700
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