Document window opens as narrow strip, will not expand to full width
Every document. Every time.
Multiple documents stack inside this narrow window.
Documents set to open in tabs.
Same regardless if Illustrator is opened first or opens with document.
Documents on external drives and local machine.
Illustrator 23.0.4
2016 MacBook Pro 15-inch 16 GB RAM
Mid-2015 MacBook Pro Retina 15-inch 16 GB RAM
MacOS Mojave (current)
Mac Illustrator CC 2019 (current)
Application Frame turned off. (with frame turned on things work differently... fine)
Both built in display and 32-inch 4k.
Only started this when I upgraded to CC 2019.
Clicking here expands the window too it's original, correct size.
Reset preferences.
Uninstalled Illustrator and files. Caches, etc.
Reset workspaces. Changed Workspaces.
Happens on both of my machines. I do not run any third-party plugins or extensions, so all is basically stock across machines.

-
Derek commented
With Application Frame turned off.
When I save a document and its document window is wider than approximately 3200 pixels, the subsequent oping of that document will only expand the document window to about 250 pixels or so.
If I reduce the width of a document window to less than about 3200 pixels and save. When oping the document it will open expanded to its full width as when saved.
Happens regardless display resolution. Although it is obviously more of an issue on 4k displays because you are more likely to get a document window wider than 3200 pixels.
OS and Illustrator version does not matter. Happens old and new. Tried reporting this 2 years ago. Cannot speak to Windows version. I have duplicated this on every Mac I can with Illustrator.
Tech support has been useless helping out. All they want to do is reset preferences and reinstall app.
MacOS 11.4
Illustrator 25 -
Derek commented
Just checking in.
Here it is late 2020.
Upgraded to latest MacOS (Catalina). Updated to latest Adobe CC apps (Illustrator, InDesign, Photoshop, etc.). Why not... I pay for it monthly.
The Illustrator document window still opens as a narrow strip every time with application frame turned off.