How do I make the active window automatically expand to take up most of the screen?

Explanation

Setting winheight to a very large number forces Vim to always try to make the focused window as tall as possible. Because Vim must also satisfy winminheight for every other window, the result is a natural "accordion" layout: the active split expands to nearly fill the screen while inactive splits collapse to just a few lines.

How it works

• winheight=999 — the minimum height Vim will try to give the current window. Since 999 lines is almost always more than the terminal height, Vim gives it all available space.
• winminheight=5 — every other window gets at least 5 lines, enough to see the filename and a few lines of context
• Vim recalculates layout every time you move focus to a different window, so the zoom effect is automatic

Add the same pair for width to get zoom in both dimensions:

set winheight=999 winminheight=5
set winwidth=999 winminwidth=15

Example

Before (equal splits):
  ┌────────────────┐
  │ file1.py (10L) │
  ├────────────────┤
  │ file2.py (10L) │
  └────────────────┘

After focusing file1.py with winheight=999:
  ┌────────────────┐
  │                │
  │ file1.py (18L) │
  │                │
  ├────────────────┤
  │ file2.py  (2L) │
  └────────────────┘

Tips

• This setting persists for the session; toggle it on/off with a mapping if you want to switch between zoom and equal layouts
• :wincmd = (or <C-w>=) resets all windows to equal sizes, temporarily overriding the auto-zoom until you move focus again
• :help 'winheight' for full details