More floating window rendering issues

[sid-6581]

In mini.files the help window is transparent (edit: it's only transparent with respect to the underlying float, not the main buffer):

In neo-tree the preview window isn't transparent, but the contents are off by one, where the first line is rendered one line too high, and the bottom content line is blank:

Desktop (please complete the following information):

• OS: Windows 11
• Neovide Version: Latest commit
• Neovim Version: Nightly

[fredizzimo]

I'm commenting on the first issue first, and will investigate the second one next.
The windows have z-index 99 and 100 respectively, and with:

• feat: render same z-index together #2467
  It treats those as one layer, so they become transparent with respect to each other. This was something I was afraid would happen when I made this comment:
• feat: render same z-index together #2442 (comment)
  And we had more discussion about it on Discord here https://discordapp.com/channels/896132176091955261/896263985614168125/1225611427495284776

There are basically two options, I think.

1. Standardize that consecutive z-indices are special around the whole nvim echo system. The issue can be fixed by using z-index 100 and 102 for example.
2. Properly experiment, if we actually need to combine different layers, or if it's enough to use a single layer while still maintaining compatibility with the terminal mode rendering. This probably also requires some adaptation on the plugin side, but if it works, I think it's a much nicer option than number 1.

@Kethku

[fredizzimo]

For the second issue, I'm not 100% sure yet, but almost. The preview window consists of two subwindows, the first one has this anchor

TRACE [neovide::channel_utils] neovim_handler WindowFloatPosition { grid: 10, anchor: NorthWest, anchor_grid: 1, anchor_row: -1.0, anchor_column: 40.0, focusable: false, sort_order: Some(39) }

While the second one has this:

TRACE [neovide::channel_utils] neovim_handler WindowFloatPosition { grid: 11, anchor: NorthWest, anchor_grid: 10, anchor_row: 1.0, anchor_column: 1.0, focusable: true, sort_order: Some(40) }

So, the second one is attached to the first one, but the first one is actually anchored outside the screen. Neovim does not reposition floats that are outside the grid, so we are forced to do it ourselves:

• The position of floating windows is not updated when multigrid is used neovim/neovim#24801

But I think our code is a bit too naive and only consider each window individually. Therefore, it only re-positions the outer window with the borders, leaving the inner one in place, which means that they start to overlap. I think we just need to consider all the attached floating windows as a whole group and reposition it as a whole.

[sid-6581]

@fredizzimo after looking at it some more, I think the second issue may be a nui bug. neo-tree is requesting a popup with row 0, and then nui adjusts it to -1 here:

https://github.com/MunifTanjim/nui.nvim/blob/cbd2668414331c10039278f558630ed19b93e69b/lua/nui/popup/border.lua#L342-L347

I'm honestly not sure if this is something neovim should do something about when multigrid is enabled? It does appear to clamp the values in the terminal, so it is technically inconsistent, but I'm not sure if the way neovim/neovide handles it now is really wrong.