make sure the bottom of the data stays at the bottom (#51)

in large data, getting close to the bottom would make the last element
"lift off" the bottom of the frame, leaving empty lines below it even
though there is enough lines to fill the entire frame...

this PR constrains the "tops" to be less than `nb_lines - height`,
forcing the last element of the data to always be at the bottom if there
are enough lines to fill the frame.

src/ui.rs

@@ -333,11 +333,15 @@ fn render_data(frame: &mut Frame, app: &mut App, config: &Config) {
 
     if cursor >= top + height - margin {
         app.rendering_tops.pop();
-        app.rendering_tops
-            .push((cursor - height + margin + 1).max(0));
+        app.rendering_tops.push(
+            (cursor - height + margin + 1)
+                .min(nb_lines as i32 - height)
+                .max(0),
+        );
     } else if cursor <= top + margin {
         app.rendering_tops.pop();
-        app.rendering_tops.push((cursor - margin).max(0));
+        app.rendering_tops
+            .push((cursor - margin).min(nb_lines as i32 - height).max(0));
     }
 
     let margin_offset = *app.rendering_tops.last().unwrap_or(&0) as usize;