Fix scroll_to_row not taking the header into account (#44)

Title. 

Also update the demo to test/verify scrolling to specific rows/columns.

Author: abey79

demo/src/table_demo.rs

@@ -301,17 +301,23 @@ impl TableDemo {
             self.prefetched.clear();
         });
 
+        let first_scrollable_col = self.num_sticky_cols;
+        let last_col = self.num_columns.saturating_sub(1);
+        let mid_col = (first_scrollable_col + last_col) / 2;
         let mut scroll_to_column = None;
         ui.horizontal(|ui| {
             ui.label("Scroll horizontally to…");
             if ui.button("left").clicked() {
-                scroll_to_column = Some(0);
+                scroll_to_column = Some(first_scrollable_col);
             }
             if ui.button("middle").clicked() {
-                scroll_to_column = Some(self.num_columns / 2);
+                scroll_to_column = Some(mid_col);
             }
             if ui.button("right").clicked() {
-                scroll_to_column = Some(self.num_columns.saturating_sub(1));
+                scroll_to_column = Some(last_col);
+            }
+            if ui.button(format!("column {mid_col}")).clicked() {
+                scroll_to_column = Some(mid_col);
             }
         });
 
@@ -327,6 +333,10 @@ impl TableDemo {
             if ui.button("bottom").clicked() {
                 scroll_to_row = Some(self.num_rows.saturating_sub(1));
             }
+            let mid_row = self.num_rows / 2;
+            if ui.button(format!("row {mid_row}")).clicked() {
+                scroll_to_row = Some(mid_row);
+            }
         });
 
         ui.separator();

demo/tests/snapshots/table_demo.png

@@ -759,37 +759,34 @@ impl SplitScrollDelegate for TableSplitScrollDelegate<'_> {
             let mut target_align = None;
 
             if let Some((column_range, align)) = &self.table.scroll_to_columns {
+                // Use the first scrollable column as the base, so that offsets start
+                // at 0 for the first non-sticky column — mirroring how row_top_offset
+                // starts at 0 for the first data row.
+                let scrollable_col_x_base = self.col_x[self.table.num_sticky_cols];
                 let x_from_column_nr = |col_nr: usize| -> f32 {
-                    let mut x = self.col_x[col_nr];
-
-                    let sitcky_width = self.col_x[self.table.num_sticky_cols] - self.col_x.first();
-                    if x < sitcky_width {
-                        // We need to do some shenanigans here because how the `SplitScroll` works:
-                        x -= sitcky_width;
-                    }
-
-                    ui.min_rect().left() + x
+                    ui.min_rect().left() + (self.col_x[col_nr] - scrollable_col_x_base)
                 };
 
-                target_rect.min.x = x_from_column_nr(*column_range.start());
+                let sticky_width = scrollable_col_x_base - self.col_x.first();
+
+                // Subtract sticky_width from the left of the target rect so that when
+                // scroll_to_rect aligns the left of the target to the viewport left, the
+                // actual column lands just right of the sticky columns (not behind them).
+                target_rect.min.x = x_from_column_nr(*column_range.start()) - sticky_width;
                 target_rect.max.x = x_from_column_nr(*column_range.end() + 1);
                 target_align = target_align.or(*align);
             }
 
             if let Some((row_range, align)) = &self.table.scroll_to_rows {
-                let y_from_row_nr = |row_nr: u64| -> f32 {
-                    let mut y = self.get_row_top_offset(row_nr);
+                let y_from_row_nr =
+                    |row_nr: u64| -> f32 { ui.min_rect().top() + self.get_row_top_offset(row_nr) };
 
-                    let sticky_height = self.header_row_y.last() - self.header_row_y.first();
-                    if y < sticky_height {
-                        // We need to do some shenanigans here because how the `SplitScroll` works:
-                        y -= sticky_height;
-                    }
-
-                    ui.min_rect().top() + y
-                };
+                let sticky_height = self.header_row_y.last() - self.header_row_y.first();
 
-                target_rect.min.y = y_from_row_nr(*row_range.start());
+                // Subtract sticky_height from the top of the target rect so that when
+                // scroll_to_rect aligns the top of the target to the viewport top, the
+                // actual row lands just below the sticky header (not behind it).
+                target_rect.min.y = y_from_row_nr(*row_range.start()) - sticky_height;
                 target_rect.max.y = y_from_row_nr(*row_range.end() + 1);
                 target_align = target_align.or(*align);
             }