When you press the Up or Down key to move the cursor in a text area in Microsoft Edge, the cursor doesn't move correctly. For example, when you press the Up key, the cursor may jump to the beginning of the line instead of moving up a line. When you press the Down key, the cursor may move down two lines instead of moving down to the next line.
This problem only occurs in the initial version of Windows 10 that is Windows 10 version 1511.
To resolve this problem, install the Windows 10 Anniversary Update (version 1607) or the Windows 10 Creators Update (version 1703).
Microsoft has confirmed this issue in the product listed in the "Applies To" section.