Well implemented user interfaces have polish that makes their inherent complexity invisible, but polish is actually millions of tiny little scratches, not just a clean simple perfectly flat surface.

Accessibility and internationalization are two crucial dimensions that most people forget about (especially non-sensory/motor-impaired Americans), which each add huge amounts of unavoidable complexity to text fields and the rest of the widget set.

Then there's text selection, character/word/paragraph level selection, drag-n-drop, pending delete, scrolling, scrollbar hiding, auto scroll, focus management, keyboard navigation and shortcuts, copy and paste, alternative input methods, type-ahead, etc, all which need to perfectly dovetail together (like auto-scrolling working correctly during selection and drag-n-drop, auto-scrolling triggering on a reasonably scaled timer regardless of mouse position instead of mouse movements only inside the text field, so scrolling is deterministically controllable and happens at a reasonable speed, and doesn't freeze when you stop moving the mouse or move it too far, etc).

There are so many half-assed custom text fields out there written by well intentioned people who just didn't realize the native text fields supported all those features, or weren't intimately familiar with all of the nuances and tweaks that have been hashed out over the decades (like anchoring and extending the selection, controlling and editing the selection with the keyboard, inserting and removing redundant spaces at the seams of the beginning and the end of the selection when you drag and drop text, etc).

Even when somebody achieves the straightforward task of implementing a text field that looks pixel-for-pixel equivalent to a native text field, they're usually making a promise that they can't keep, that it also operates exactly the same as a native text field.

I've seen many text fields in games (and browsers) that break type-ahead by dropping keyboard input when you type too fast, because instead of tracking device events in a queue, they're polling the current state of the keys each frame update, so when you get slow frames and stuttering (which is often, like during auto save or browser thrashing), they miss key transitions.

Most games poll the mouse buttons and positions this way too, so they break mouse-ahead by dropping mouse clicks if you make them too fast, and they perform actions at the current position of the mouse instead of its position when the click happened.

Even a beautifully designed well implemented AAA quality game like Dyson Sphere Project running on a high-end PC has this problem. After you place a power pole, you have to hold the mouse still for a moment to let the game handle the mouse down event and draw the screen a few times, before daring to move your mouse away from where you want to place the pole, otherwise the pole goes into the wrong position, away from where you clicked the mouse, and this really throws a monkey wrench into smooth fluid interaction, predictable reliability, mouse-ahead, etc.

The Xerox Star had a wonderfully well thought out and implemented text editor, which pioneered solutions to many of these issues in 1982 (including internationalization), demonstrated in this video:

Xerox Star User Interface (1982) 2 of 2

https://www.youtube.com/watch?v=ODZBL80JPqw

See Brad Myers video "All the Widgets (Fixed v2) - 1990". This was made in 1990, sponsored by the ACM CHI 1990 conference, to tell the history of widgets up until then. Previously published as: Brad A. Myers. All the Widgets. 2 hour, 15 min videotape. Technical Video Program of the SIGCHI'90 conference, Seattle, WA. April 1-4, 1990. SIGGRAPH Video Review, Issue 57. ISBN 0-89791-930-0.

https://www.youtube.com/watch?v=9qtd8Hc90Hw

Also by Brad Myers:

Taxonomies of Visual Programming (1990) [pdf] (cmu.edu)

https://news.ycombinator.com/item?id=26057530

https://www.cs.cmu.edu/~bam/papers/VLtax2-jvlc-1990.pdf

Updated version:

http://www.cs.cmu.edu/~bam/papers/chi86vltax.pdf

Brad Myers is finishing a book (tentatively titled “Pick, Click, Flick! The Story of Interaction Techniques”) which is partially a history of Interaction Techniques. Probably more than 450 pages. The initial chapter list can be seen at www.ixtbook.com. It is based on Brad’s All The Widgets video and Brief History of HCI paper, and also on his class on Interaction Techniques which he taught three times. As part of that class, Brad interviewed 15 inventors of different interaction techniques, all but one of whose video is available on-line, which also might be a useful resource.

Pick, Click, Flick! The Story of Interaction Techniques:

http://www.ixtbook.com/

https://www.cs.cmu.edu/~bam/ixtbook/#abstract

Brad Myers' Interaction Design Class:

https://www.cs.cmu.edu/~bam/uicourse/05440inter/

Here's the video and slides of the talk I gave to Brad's Interaction Techniques class about pie menus -- there's a discussion of mouse ahead, event handling, and polling around 16:30:

Video:

https://scs.hosted.panopto.com/Panopto/Pages/Viewer.aspx?id=...

Slides:

https://docs.google.com/presentation/d/1R9s4EEAwUjI_7A8GgdLY...

Pie Menus: A 30 Year Retrospective (Timeline):

https://medium.com/@donhopkins/pie-menus-936fed383ff1