wrong scaling for external HD monitor using VGA, MBP 13" I know this sounds similar to other questions but bear with me - I've investigated and am not finding any solutions. Sorry in advance for a lengthy question.

I have a MacBook Pro (13-inch, 2016, Two Thunderbolt 3 ports.) Since I purchased it in summer 2016 I've used it with a couple of different cheap HD monitors by Samsung, connected with the Apple thunderbolt-VGA multi-adapter. This has never been an issue.

A few days ago, I cracked the screen. (That's another story.) I'm a developer and have to keep using this machine heavily for the moment (My backup 2012 iMac would have a hard time keeping up with the current tasks.) It continued to work just fine in "clamshell" mode with an external mouse and keyboard. 

This evening though, I opened the MBP lid to see if I could access its camera. Suddenly, the external display resolution defaulted to 800x600. The "scaled" options in Display preferences now allow for 800x600 and 1280x1024 only. 

Only by option-clicking the "scaled" radio button in Display preferences can I access the monitor's native resolution of 1920x1080 (this has never been necessary before.)

(and now the meat of the question:) When I do select the full HD resolution, the image is almost correct, but asymmetrically narrowed - leaving a black bar of, I don't know, maybe 120px on the left side and 40px on the right. It's almost usable, but causes a definite headache.

Moreover, the exact same problem occurs with a different monitor! It certainly seems like a software/firmware issue on the MBP itself.

Has anyone else experienced this?
Could it be somehow due to the cracked Retina display? (If so, how??)

Things I've tried:


resetting the SMC
resetting the PRAM
disconnecting all peripherals besides the VGA adapter


Things I haven't tried:


booting into safe mode (I'm worried I'll be stuck not seeing anything at all on the external display.)
using an HDMI adapter (I never bought one for this machine.)
swapping the VGA cable (will do)


I'm a bit at a loss and would appreciate any ideas or reports of similar behavior!
ok, replacing the VGA cable worked.

and - apologies - it appears to be the same issue/solution reported here:
External monitor randomly lowers resolution adds black borders

and here:
External monitor viewport appear shrunk in native resolution on MBP