ElyTable: a cell scrolls in by the least amount; a drag never scrolls under the pointer (T-02)

crates/ely-lok/src/view.rs

@@ -375,8 +375,9 @@ impl LokView {
         }
         if batch.cursor != self.cursor {
             self.moves += 1;
-            // A moved cursor stays in view.
+            // A moved cursor stays in view, but not mid-drag.
             if self.viewport.1 > 0.0
+                && !self.dragging
                 && let Some(cursor) = batch.cursor
             {
                 self.show(cursor, cx);
@@ -385,6 +386,7 @@ impl LokView {
         // A moved cell cursor stays in view too.
         if batch.cell.cursor != self.cell.cursor
             && self.viewport.1 > 0.0
+            && !self.dragging
             && let Some(cell) = batch.cell.cursor
         {
             self.show(cell, cx);

crates/ely-lok/src/view/scroll.rs

@@ -61,11 +61,11 @@ impl LokView {
             let need = self.print_of((x + w + vw + fx, y + h + vh + fy));
             self.reach = (self.reach.0.max(need.0), self.reach.1.max(need.1));
         }
-        if x >= fx && (x < fx + self.scroll.0 || x + w > fx + self.scroll.0 + vw) {
-            self.scroll.0 = x - fx - vw / 2;
+        if x >= fx {
+            self.scroll.0 = least(x - fx, w, self.scroll.0, vw);
         }
-        if y >= fy && (y < fy + self.scroll.1 || y + h > fy + self.scroll.1 + vh) {
-            self.scroll.1 = y - fy - vh / 3;
+        if y >= fy {
+            self.scroll.1 = least(y - fy, h, self.scroll.1, vh);
         }
         self.clamp_scroll();
         cx.notify();
@@ -172,3 +172,10 @@ impl LokView {
         })
     }
 }
+
+/// The scroll nearest `scroll` that shows `start..start + len`.
+fn least(start: i32, len: i32, scroll: i32, view: i32) -> i32 {
+    // A span wider than the view keeps covering it.
+    let end = start + len - view;
+    scroll.clamp(start.min(end), start.max(end))
+}

crates/ely-lok/src/view/tests/large.rs

@@ -202,6 +202,16 @@ fn a_sheet_brings_far_cut_and_frozen_cells_into_view() {
             let cut = (width - 200, 0, 1280, 256);
             view.show(cut, cx);
             assert!(view.scroll.0 > 0 && shows(view, cut), "a cell cut at the right edge comes into view");
+            let (x, _, w, _) = view.tile_rect(cut);
+            assert_eq!(view.scroll.0, x + w - width, "it scrolls no further than the cell's edge");
+            let before = view.scroll;
+            view.show((0, 0, view.print_of((x + w + width, 0)).0, 256), cx);
+            assert_eq!(view.scroll, before, "a cell wider than the view and covering it keeps still");
+            view.dragging = true;
+            let cell = crate::Cell { cursor: Some((1418, 90_000, 1280, 277)), ..crate::Cell::default() };
+            view.apply_batch(crate::Events { cell, ..crate::Events::default() }, None, cx);
+            assert_eq!(view.scroll, before, "a drag does not scroll under the pointer");
+            view.dragging = false;
             view.scroll = (0, 0);
             view.set_frozen((0, 600), cx);
             let frozen = view.place((0, 300)).y;