Update: Fallout version 3.1—in beta as of this writing—resolves this problem (hallelujah). If you're still experiencing this problem, and if 1.3 is still in beta, you can install the beta via Steam by following these instructions.
While in Workshop mode, pressing the "F" key will rotate the object you're trying to build and place. The key for this action isn't mappable through the in-game UI.
That works fine with the default key mapping. But I always remap my movement keys to ASDF, so in Workshop mode I can't move forward. I've been site-stepping and backing up to get around town.
This is just one example of a possible conflict with unmappable keys. The "E" key seems to be a source of problems too.
Is it possible to remap keys if their mappings aren't exposed through the UI? On the PC there's an intriguingly named file at
G:\Documents\My Games\Fallout4\ControlMap_Custom.txt
but its contents are pretty inscrutable. And there were a couple of Skyrim mods for remapping keys, but it's early for the FO4 community to have one.
Best Answer
You can use AutoHotkey to remap keys. Obviously this isn't an ideal solution since you have to install something and remember to turn the script on and off, but it should work until a mod comes up.