For more on this, the book:

https://www.goodreads.com/book/show/483933.ThinkPad

is highly recommended.

Also, Jerry Kaplan's wonderful:

https://www.goodreads.com/book/show/1171250.Startup

touches a bit on the early history of it as a pen computer project.

Still have my ThinkPad 755c, and still a bit bummed that I was never able to get OpenSTEP to run on it. (It's worth noting that when David Pogue complained of Steve Jobs using a ThinkPad after the purchase of NeXT by Apple that it was running NeXTstep and not Windows 95 as he claimed.)

Really wish the Lenovo Yogabook 9i was a ThinkPad model (so that it would have a Trackpoint) and that it used Wacom EMR stylus technology --- wouldn't've been able to resist that.