widgets: add Scrollbar widget

src/widgets.zig

@@ -1,5 +1,6 @@
 pub const border = @import("widgets/border.zig");
 pub const alignment = @import("widgets/alignment.zig");
-pub const TextInput = @import("widgets/TextInput.zig");
+pub const Scrollbar = @import("widgets/Scrollbar.zig");
 pub const Table = @import("widgets/Table.zig");
+pub const TextInput = @import("widgets/TextInput.zig");
 pub const nvim = @import("widgets/nvim.zig");

src/widgets/Scrollbar.zig

@@ -0,0 +1,34 @@
+const std = @import("std");
+const vaxis = @import("../main.zig");
+
+const Scrollbar = @This();
+
+/// character to use for the scrollbar
+character: vaxis.Cell.Character = .{ .grapheme = "▐", .width = 1 },
+
+/// style to draw the bar character with
+style: vaxis.Style = .{},
+
+/// index of the top of the visible area
+top: usize = 0,
+
+/// total items in the list
+total: usize,
+
+/// total items that fit within the view area
+view_size: usize,
+
+pub fn draw(self: Scrollbar, win: vaxis.Window) void {
+    // don't draw when 0 items
+    if (self.total < 1) return;
+
+    // don't draw when all items can be shown
+    if (self.view_size >= self.total) return;
+
+    var bar_height = self.view_size * win.height / self.total;
+    if (bar_height < 0) bar_height = 1;
+    const bar_top = self.top * win.height / self.total;
+    var i: usize = 0;
+    while (i < bar_height) : (i += 1)
+        win.writeCell(0, i + bar_top, .{ .char = self.character, .style = self.style });
+}