Windowed mode was broken at launch. It behaved like a borderless window locked to the center of the screen, and there was no way to drag it. Thank you to everyone who reported it you were right, and it's fixed.
Fixed
Windowed mode now uses a proper window frame. You can drag it, resize it, and snap it with Win + arrow keys like any other window.
The window no longer jumps back to the center when you change display mode or resolution.
Your window position and size are now remembered between sessions.
Windowed resolutions are capped to fit your desktop, so the window can never open larger than your screen.
If Windows restores the window from maximized, the options menu now updates to match instead of still showing "Maximized".
"Reset to defaults" now actually applies the display mode instead of only changing the label.
Changed
First launch now defaults to Fullscreen, which is what the game was already doing. If you've already picked a display mode, your choice is kept.
Note
Dragging a maximized window restores it to a smaller size. That's standard Windows behavior you can maximize it again from the window edge or button.
This one was a basics failure on our part, and the feedback was fair. Found something else? Post it in the discussions we read them.