Extra Character Row for full Screen TUIs ("Scroll 0/1")

[KyleKing]

Description

When launching a full screen TUI in Zellij, the screen itsefl scrolls, when it should have a 100% fill. This doesn't appear to be terminal emulator or library-specific because I tried TUI applications implemented in several languages (bpytop, lazygit, htop, etc.) in iTerm2 and Alacritty

Further information

To reproduce, launch zellij, launch any fullscreen TUI (i.e. lazygit or htop), then attempt to scroll and you should see the whole interface move up or down one row when it instead should fill the screen and not move

Demo with asciinema:

Logs from testing with lazygit:

zellij-11.log
zellij.log

Device information

zellij --version: zellij 0.22.1
tput lines: 58
tput cols: 192
uname -av or ver(Windows): Darwin MacBook-Pro.local 19.6.0 Darwin Kernel Version 19.6.0: Sun Nov 14 19:58:51 PST 2021; root:xnu-6153.141.50~1/RELEASE_X86_64 x86_64
alacritty --version: alacritty 0.9.0 (fed349a)
lazygit --version: commit=, build date=, build source=homebrew, version=0.31.4, os=darwin, arch=amd64
(Reproducible in latest versions of iTerm2 or Kitty and with htop, bpytop, etc.)

Experimentation

I tried: zellij options --no-pane-frames and/or the most generic layout possible and the error persisted:

---
template:
  direction: Horizontal
  parts:
    - direction: Vertical
      no_pane_frames: true

[KyleKing]

This is a blocker for me, so tried to take a look in more depth

This regression appears to be introduced in Zellij 0.18 and isn't present in 0.17 or 0.16. I think it might be related to this change (fb8e44b), but I don't know Go (Rust 😆) or the codebase. This was the full diff between releases: 1868816...50452b5

I'm trying out a fork now locally, but not sure how far I can get

[KyleKing]

Okay, so reverting fb8e44b by restoring self.viewport.clear() fixes this issue, but is counter to the changes requested in #743. Maybe there needs to be separate logic for a full viewport clear vs. only clear-screen.

Relevant function:

zellij/zellij-server/src/panes/grid.rs

Lines 909 to 926 in 0863e74

	pub fn fill_viewport(&mut self, character: TerminalCharacter) {
	let row_count_to_transfer = self.viewport.len();
	let transferred_rows_count = transfer_rows_from_viewport_to_lines_above(
	&mut self.viewport,
	&mut self.lines_above,
	row_count_to_transfer,
	self.width,
	);
	self.scrollback_buffer_lines =
	subtract_isize_from_usize(self.scrollback_buffer_lines, transferred_rows_count);
	for _ in 0..self.height {
	let columns = VecDeque::from(vec![character; self.width]);
	self.viewport.push(Row::from_columns(columns).canonical());
	}
	self.output_buffer.update_all_lines();
	}

Regardless, there is still an issue with scrolling that prevents use of most TUIs (which I think is being addressed in #624)

[tlinford]

@KyleKing you were spot on, hope to get this sorted pretty soon

[tlinford]

fixed in #1032