wezterm/wezterm-gui/src/scrollbar.rs

use ::window::*;
use mux::pane::Pane;
use portable_pty::PtySize;
use wezterm_term::StableRowIndex;

pub enum ScrollHit {
    Above,
    OnThumb(isize),
    Below,
}

pub struct ThumbInfo {
    /// Offset from the top of the window in pixels
    pub top: usize,
    /// Height of the thumb, in pixels.
    pub height: usize,
    /// Number of rows that correspond to the thumb in rows.
    /// This is normally == viewport height, but in the case
    /// where there are a sufficient number of rows of scrollback
    /// that the pixel height of the thumb would be too small,
    /// we will scale things in order to remain useful.
    pub rows: usize,
}

impl ScrollHit {
    /// Given a mouse y value, determine whether the cursor is above, over
    /// or below the thumb.
    /// If above the thumb, return the offset from the top of the thumb.
    pub fn test(
        y: isize,
        pane: &dyn Pane,
        viewport: Option<StableRowIndex>,
        size: PtySize,
        dims: &Dimensions,
    ) -> Self {
        let info = Self::thumb(pane, viewport, size, dims);
        if y < info.top as isize {
            Self::Above
        } else if y < (info.top + info.height) as isize {
            Self::OnThumb(y - info.top as isize)
        } else {
            Self::Below
        }
    }

    /// Compute the y-coordinate for the top of the scrollbar thumb
    /// and the height of the thumb and return them.
    pub fn thumb(
        pane: &dyn Pane,
        viewport: Option<StableRowIndex>,
        size: PtySize,
        dims: &Dimensions,
    ) -> ThumbInfo {
        let render_dims = pane.get_dimensions();

        let scroll_top = render_dims
            .physical_top
            .saturating_sub(viewport.unwrap_or(render_dims.physical_top));

        let scroll_size = render_dims.scrollback_rows;

        let thumb_size = (size.rows as f32 / scroll_size as f32) * dims.pixel_height as f32;

        const MIN_HEIGHT: f32 = 10.;
        let (thumb_size, rows) = if thumb_size < MIN_HEIGHT {
            let scale = MIN_HEIGHT / thumb_size;
            let rows = size.rows as f32 * scale;
            (MIN_HEIGHT, rows as usize)
        } else {
            (thumb_size, size.rows as usize)
        };

        let thumb_top = (1. - (scroll_top + rows as StableRowIndex) as f32 / scroll_size as f32)
            * size.pixel_height as f32;

        let thumb_size = thumb_size.ceil() as usize;
        let thumb_top = thumb_top.ceil() as usize;

        ThumbInfo {
            top: thumb_top,
            height: thumb_size,
            rows,
        }
    }

    /// Given a new thumb top coordinate (produced by dragging the thumb),
    /// compute the equivalent viewport offset.
    pub fn thumb_top_to_scroll_top(
        thumb_top: usize,
        pane: &dyn Pane,
        viewport: Option<StableRowIndex>,
        size: PtySize,
        dims: &Dimensions,
    ) -> StableRowIndex {
        let render_dims = pane.get_dimensions();
        let thumb = Self::thumb(pane, viewport, size, dims);

        let rows_from_top = ((thumb_top as f32) / thumb.height as f32) * thumb.rows as f32;

        render_dims
            .scrollback_top
            .saturating_add(rows_from_top as StableRowIndex)
    }
}