Your Dock, the size you set it.

The divider is a trap

Look at the thin vertical line between your pinned apps and the Recents / Files / Trash group. On a horizontal Dock it sits right in the middle of everything you click all day; on a vertical Dock it's near the bottom. It is also a resize handle, and it's about as wide as the icons on either side of it.

So the failure mode is ordinary: you go to click an app, catch the divider by a few pixels, and drag. macOS doesn't ask whether you meant it. There's no confirmation, no modifier key to hold, no haptic feedback marking the boundary. The Dock simply resizes, and now every icon you had muscle memory for is somewhere else.

Why System Settings can't fix it

Open System Settings → Desktop & Dock and you'll find sliders for size and magnification — but nothing that disables dragging. The handle isn't a feature you opted into; it's part of how the Dock works, and Apple ships no switch for it.

The one mitigation macOS offers is right-clicking the divider to choose a preset size. That helps you recover after a mis-drag, but it does nothing to prevent the next one. The handle is still live, still the same width, still sitting between the icons you click most.

What Lock Dock Size does

Flip the lock and the divider stops responding to drags. Your tile size and magnification are pinned to whatever you had when you turned it on, and they stay pinned — through app launches, display changes, sleep and wake.

Everything else about the Dock keeps working normally. You can still click and launch apps, right-click for app menus, use the Recents group, and drag files onto Dock icons. The only interaction that changes is the one you never wanted: resizing.

When you do want a different size, toggle the lock off, adjust, and toggle it back on. Quitting DockLockr releases the lock cleanly too — it never leaves your Dock stuck.

Turning it on

1. Set your Dock to the size and magnification you actually want.
2. Click the DockLockr lock icon in your menu bar.
3. Switch on Lock Dock Size.

If your Dock also gets rearranged by stray drags, pair this with Lock Dock Items. The two are independent toggles and work well together.