What?

⌘ + ↑ / ⌘ + ↓ on macOS is a common convention to move the cursor to the very start or end of the document. In Gutenberg, this shortcut was not working correctly — it only moved the caret to the top/bottom of the current block, not the first/last block in the document.
Closes #78919.

Why?

The useArrowNav hook in the writing-flow intercepts vertical arrow key presses when the caret is at a block edge and moves focus to the adjacent block. However, it never handled the special case of metaKey + vertical arrow. This allowed the browser's default behaviour to run, which only moves within the current contenteditable element.

How?

• Added a getEdgeTabbable( containerElement, isReverse ) helper function that finds the absolute first or last tabbable element in the writing-flow container using the same candidate criteria as getClosestTabbable.
• In onKeyDown, added a new branch: when metaKey && isVertical && !keepCaretInsideBlock, we call getEdgeTabbable to get the target and placeCaretAtVerticalEdge to position the cursor, then preventDefault() to skip the browser's native handling.

Testing Instructions

1. Open a post in the Gutenberg editor.
2. Add several paragraph blocks with content.
3. Click somewhere in a middle block.
4. Press ⌘ + ↑ — cursor should jump to the beginning of the first block.
5. Press ⌘ + ↓ — cursor should jump to the end of the last block.

Screenshots or screencast

Before:

After: