Grid view: Linked scrolling with arrow keys

[johnwdubois]

Describe the bug
When in grid view, the two grids are only linked (that is, they scroll together in sync) when you drag the scroll bar. They should also be linked when you just use the up/down arrows in one of the two grids, same as with the scroll bar.

To Reproduce
Steps to reproduce the behavior:

1. Go to grid view (with 'g')
2. Click on 'unlink'
3. Click on 2 different grids (e.g. unit grid on left, line grid on right).
4. Put the cursor into the grid on the right.
5. Scroll down, so that the two grids are not correlated.
6. Now click on the 'link' icon to link them.
7. Put cursor in the grid on the right. Use arrow keys to scroll up and down. Notice that the left grid does not scroll in sync with the right.
8. Grab the scroll bar and drag it up and down. Notice that the left grid scrolls nicely, in sync.

Expected behavior
We would like the two grids to scroll together in sync, whether you use the scroll bar or the arrow keys. Make the arrow keys work like the scroll bar.

Desktop (please complete the following information):

• OS: Windows 10