Problem:
a ghost cmdline appears when scrolling a full-height float window downward

Solution:
Change bot - MAX(rows, 0) to bot - 1 to correctly check if the float grid is being covered by msg_grid

will fix #34512

Additional notes:

1. bot - 1 could also correctly detect if default_grid is covered by msg_grid when laststatus=0, this avoids triggering a recompose:

   v0.11.3	this PR
   v0.11.3.mp4	this-pr.mp4

2. bot - MAX(rows, 0) is introduced in PR #11025, and I also add a test case for its related issue #11019.
3. BTW, a later PR #11121 ensures that attrbuf[i] > 0, so I feel the condition code below might be outdated. If we want to remove the code later, the test case might be useful.
   

   neovim/src/nvim/ui_compositor.c

   Lines 679 to 686 in 1cb1cfe

   	// TODO(bfredl): workaround for win_update() performing two scrolls in a
   	// row, where the latter might scroll invalid space created by the first.
   	// ideally win_update() should keep track of this itself and not scroll
   	// the invalid space.
   	if (curgrid->attrs[curgrid->line_offset[r - curgrid->comp_row]
   	+ (size_t)left - (size_t)curgrid->comp_col] >= 0) {
   	compose_line(r, left, right, 0);
   	}

This is the 4th issue I dug into. I picked up a lot of low-level concepts while debugging last week, but there might still be gaps in my understanding. Please feel free to point out anything if I'm wrong. :)