account for number of floating bottom rows in lastTop

[spasovski]

fixes #299

This seems to work as expected now.

[mofojed]

This isn't right... so the params passed in to getLastTop include visibleHeight, which should be the height of the "visible" viewport (for lack of a better name), ie. everything that's not floating. So visually:

|-------------------------------|
| floatingTopRowHeight |
|-------------------------------|
|                                     |
|       visibleHeight          |
|                                     |
|-------------------------------|
| floatingBottomRowHeight |
|-------------------------------|

This algorithm is looking for the lastTop, which is the row that is at the top when scrolled so the last row is visible.
When we have floating bottom rows, we need to make sure the last non-floating row is visible, so we start at let lastTop = Math.max(0, rowCount - floatingBottomRowCount - 1); (last non-floating row), and iterate backward adding the height of each row until we exceed the visibleHeight, and then return the row after that (eg. lastTop + 1 in this case). Adding floatingBottomRowCount doesn't make sense - if floatingBottomRowCount is 0, then you're lastTop is going to be one row higher than it should be, and you won't be able to scroll to the bottom when there's no floatingBottomRowCount. You can observe this by checking the style guide (http://localhost:4000/styleguide)

I think this algorithm is correct here (before the change), so the issue may be with the visibleHeight that's passed in...

[mofojed]

Yea - check where getLastTop is called from initializing lastTop:

    const lastTop = this.getLastTop(
      state,
      null,
      height - gridY - scrollBarSize
    );

That height - gridY - scrollBarSize isn't taking into account the floatingBottomHeight at all. Need to take that into account. I don't think you need to worry about the floatingTopRowHeight because the top scrolled row is scrolled underneath the floating top rows, but try playing around with MockGridModel by changing up floatingTop/BottomRowCount and testing in the style guide (would be nice to add controls straight into the style guide to modify stuff, but meh)