Fix: prevent panic on scrolling beyond buffer limits;

[5-pebbles]

Currently if you scroll to the bottom of the buffer, and then scroll down a popup menu helix will panic.

Error:

thread 'main' panicked at 'Trying to access position outside the buffer: x=17, y=33, area=Rect { x: 0, y: 0, width: 124, height: 33 }', helix-tui/src/buffer.rs:235:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Full Backtrace

$ ~/apps/rust/helix/target/debug/hx
thread 'main' panicked at 'Trying to access position outside the buffer: x=19, y=33, area=Rect { x: 0, y: 0, width: 124, height: 33 }', helix-tui/src/buffer.rs:235:9
stack backtrace:
   0: rust_begin_unwind
             at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/std/src/panicking.rs:578:5
   1: core::panicking::panic_fmt
             at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/core/src/panicking.rs:67:14
   2: helix_tui::buffer::Buffer::index_of
             at /home/ghostbird/apps/rust/helix/helix-tui/src/buffer.rs:235:9
   3: <helix_tui::buffer::Buffer as core::ops::index::IndexMut<(u16,u16)>>::index_mut
             at /home/ghostbird/apps/rust/helix/helix-tui/src/buffer.rs:658:17
   4: <helix_term::ui::menu::Menu<T> as helix_term::compositor::Component>::render
             at /home/ghostbird/apps/rust/helix/helix-term/src/ui/menu.rs:400:33
   5: <helix_term::ui::popup::Popup<T> as helix_term::compositor::Component>::render
             at /home/ghostbird/apps/rust/helix/helix-term/src/ui/popup.rs:319:9
   6: <helix_term::ui::completion::Completion as helix_term::compositor::Component>::render
             at /home/ghostbird/apps/rust/helix/helix-term/src/ui/completion.rs:444:9
   7: <helix_term::ui::editor::EditorView as helix_term::compositor::Component>::render
             at /home/ghostbird/apps/rust/helix/helix-term/src/ui/editor.rs:1563:13
   8: helix_term::compositor::Compositor::render
             at /home/ghostbird/apps/rust/helix/helix-term/src/compositor.rs:178:13
   9: helix_term::application::Application::render::{{closure}}
             at /home/ghostbird/apps/rust/helix/helix-term/src/application.rs:278:9
  10: helix_term::application::Application::handle_terminal_events::{{closure}}
             at /home/ghostbird/apps/rust/helix/helix-term/src/application.rs:651:26
  11: helix_term::application::Application::event_loop_until_idle::{{closure}}
             at /home/ghostbird/apps/rust/helix/helix-term/src/application.rs:320:55
  12: helix_term::application::Application::event_loop::{{closure}}
             at /home/ghostbird/apps/rust/helix/helix-term/src/application.rs:294:57
  13: helix_term::application::Application::run::{{closure}}
             at /home/ghostbird/apps/rust/helix/helix-term/src/application.rs:1194:38
  14: hx::main_impl::{{closure}}
             at /home/ghostbird/apps/rust/helix/helix-term/src/main.rs:161:53
  15: tokio::runtime::park::CachedParkThread::block_on::{{closure}}
             at /home/ghostbird/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.37.0/src/runtime/park.rs:281:63
  16: tokio::runtime::coop::with_budget
             at /home/ghostbird/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.37.0/src/runtime/coop.rs:107:5
  17: tokio::runtime::coop::budget
             at /home/ghostbird/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.37.0/src/runtime/coop.rs:73:5
  18: tokio::runtime::park::CachedParkThread::block_on
             at /home/ghostbird/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.37.0/src/runtime/park.rs:281:31
  19: tokio::runtime::context::blocking::BlockingRegionGuard::block_on
             at /home/ghostbird/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.37.0/src/runtime/context/blocking.rs:66:9
  20: tokio::runtime::scheduler::multi_thread::MultiThread::block_on::{{closure}}
             at /home/ghostbird/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.37.0/src/runtime/scheduler/multi_thread/mod.rs:87:13
  21: tokio::runtime::context::runtime::enter_runtime
             at /home/ghostbird/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.37.0/src/runtime/context/runtime.rs:65:16
  22: tokio::runtime::scheduler::multi_thread::MultiThread::block_on
             at /home/ghostbird/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.37.0/src/runtime/scheduler/multi_thread/mod.rs:86:9
  23: tokio::runtime::runtime::Runtime::block_on
             at /home/ghostbird/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.37.0/src/runtime/runtime.rs:351:45
  24: hx::main_impl
             at /home/ghostbird/apps/rust/helix/helix-term/src/main.rs:163:5
  25: hx::main
             at /home/ghostbird/apps/rust/helix/helix-term/src/main.rs:37:21
  26: core::ops::function::FnOnce::call_once
             at /rustc/90c541806f23a127002de5b4038be731ba1458ca/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

It's caused by the following code in helix-term/src/ui/menu.rs:

let render_borders = cx.editor.menu_border();

// this highlights the selected edges if there are no borders
if !render_borders {
    if let Some(cursor) = self.cursor {

        let offset_from_top = cursor - scroll;

        // this panics if area.y + offset_from_top is below the bottom of the screen
        let left = &mut surface[(area.left(), area.y + offset_from_top as u16)];

        left.set_style(selected);
        let right = &mut surface[(
            area.right().saturating_sub(1),
            area.y + offset_from_top as u16,
        )];
        right.set_style(selected);
    }
}

I've changed the logic for offset_from_top to:

let offset_from_top = (cursor - scroll).min(area.height as usize - 1).max(0);

Which seems to have fixed the problem.

Note: There could be a better way to fix this, but I don't know the codebase well enough to suggest one.

[5-pebbles]

Actually when I compiled from the latest source it works fine, so something else must have fixed it since my last install.

¯\_( -_- )_/¯

[pascalkuthe]

Probably #10573

[5-pebbles]

Yup that's it, thanks for fixing it.