layout: Offset Y animations for non-animated resizes

src/layout/scrolling.rs

@@ -4056,12 +4056,35 @@ impl<W: LayoutElement> Column<W> {
         // windows in the column, so they should all be animated. How should this interact with
         // animated vs. non-animated resizes? For example, an animated +20 resize followed by two
         // non-animated -10 resizes.
-        if !is_tabbed && tile.resize_animation().is_some() && offset != 0. {
-            for tile in &mut self.tiles[tile_idx + 1..] {
-                tile.animate_move_y_from_with_config(
-                    offset,
-                    self.options.animations.window_resize.anim,
-                );
+        if !is_tabbed && offset != 0. {
+            if tile.resize_animation().is_some() {
+                // If there's a resize animation (that may have just started in
+                // tile.update_window()), then the apparent size change is smooth with no sudden
+                // jumps. This corresponds to adding an Y animation to tiles below.
+                for tile in &mut self.tiles[tile_idx + 1..] {
+                    tile.animate_move_y_from_with_config(
+                        offset,
+                        self.options.animations.window_resize.anim,
+                    );
+                }
+            } else {
+                // There's no resize animation, but the offset is nonzero. This could happen for
+                // example:
+                // - if the window resized on its own, which we don't animate
+                // - if the window resized by less than 10 px (the resize threshold)
+                //
+                // The latter case could also cancel an ongoing resize animation.
+                //
+                // Now, stationary tiles below shouldn't react to this offset change in any way,
+                // i.e. their apparent Y position should jump together with the resize. However,
+                // tiles below that are already animating an Y movement should offset their
+                // animations to avoid the jump.
+                //
+                // Notably, this is necessary to fix the animation jump when resizing height back
+                // and forth in quick succession (in a way that cancels the resize animation).
+                for tile in &mut self.tiles[tile_idx + 1..] {
+                    tile.offset_move_y_anim_current(offset);
+                }
             }
         }
     }

src/layout/tests/animations.rs

@@ -269,14 +269,20 @@ fn height_resize_and_cancel() {
     ];
     check_ops_on_layout(&mut layout, &ops);
 
-    // Since the resize animation is cancelled, the height goes to the new value immediately, and
-    // the Y position should also go to 100 immediately, cancelling the movement.
-    //
-    // FIXME: right now, the Y position jumps to 5 and will continue animating to 100 from there
-    // because cancelling the resize anim doesn't cancel the induced Y movement.
+    // Since the resize animation is cancelled, the height goes to the new value immediately. The Y
+    // position doesn't jump, instead the animation is offset to preserve the current position.
     assert_snapshot!(format_tiles(&layout), @r"
     100 × 100 at x:  0 y:  0
-    200 × 200 at x:  0 y:  5
+    200 × 200 at x:  0 y:105
+    ");
+
+    // Advance to the end of the move animation.
+    Op::AdvanceAnimations { msec_delta: 950 }.apply(&mut layout);
+
+    // Final state.
+    assert_snapshot!(format_tiles(&layout), @r"
+    100 × 100 at x:  0 y:  0
+    200 × 200 at x:  0 y:100
     ");
 }
 
@@ -506,23 +512,30 @@ fn height_resize_and_cancel_during_another_y_anim() {
     ];
     check_ops_on_layout(&mut layout, &ops);
 
-    // Since the resize anim was cancelled, second window's Y should jump a little to go back to
-    // its original trajectory.
-    //
-    // FIXME: right now, the Y position jumps and will continue to animate from there because
-    // cancelling the resize anim doesn't cancel the induced Y movement.
+    // Since the resize anim was cancelled, second window's Y anim is adjusted to preserve the
+    // current position while targeting the new final position.
     assert_snapshot!(format_tiles(&layout), @r"
     100 × 100 at x:  0 y:  0
-    200 × 200 at x: 45 y:-43
+    200 × 200 at x: 45 y: 58
     ");
 
     // Advance the time to complete the consume movement.
     Op::AdvanceAnimations { msec_delta: 450 }.apply(&mut layout);
 
-    // Final state. Y should be at 100 since the resize-induced Y anim was cancelled.
+    // Since we don't cancel the resize-induced part of the anim (in fact the move Y anim isn't
+    // split into parts, so there's no way to tell), it keeps going still.
     assert_snapshot!(format_tiles(&layout), @r"
     100 × 100 at x:  0 y:  0
-    200 × 200 at x:  0 y: 25
+    200 × 200 at x:  0 y: 78
+    ");
+
+    // Advance the time to complete the resize-induced anim.
+    Op::AdvanceAnimations { msec_delta: 550 }.apply(&mut layout);
+
+    // Final state.
+    assert_snapshot!(format_tiles(&layout), @r"
+    100 × 100 at x:  0 y:  0
+    200 × 200 at x:  0 y:100
     ");
 }
 
@@ -723,14 +736,10 @@ fn height_resize_before_another_y_anim_then_cancel() {
     ];
     check_ops_on_layout(&mut layout, &ops);
 
-    // This should cause the second window's trajectory to readjust to the new final position at
-    // 100px. Ideally without big jumps because even though the first window's actual size changed
-    // from 200 px to 100 px, the cancelled resize only shifted it from 104 px to 100 px.
-    //
-    // FIXME: currently causes a 100 px jump due to the change in the actual window size.
+    // The second window's trajectory readjusts to the new final position at 100 px, without jumps.
     assert_snapshot!(format_tiles(&layout), @r"
     100 × 100 at x:  0 y:  0
-    200 × 200 at x: 98 y:-96
+    200 × 200 at x: 98 y:  4
     ");
 
     // Advance the time to complete the consume movement.
@@ -791,13 +800,10 @@ fn clientside_height_change_during_another_y_anim() {
     ];
     check_ops_on_layout(&mut layout, &ops);
 
-    // This should cause the second window's trajectory to readjust to the new final position at
-    // 200px. Ideally without big jumps.
-    //
-    // FIXME: currently causes a 100 px jump due to the change in the window size.
+    // The second window's trajectory readjusts to the new final position at 200 px, without jumps.
     assert_snapshot!(format_tiles(&layout), @r"
     100 × 200 at x:  0 y:  0
-    200 × 200 at x: 80 y:120
+    200 × 200 at x: 80 y: 20
     ");
 
     // Advance the time to complete the consume movement.

src/layout/tile.rs

@@ -485,6 +485,17 @@ impl<W: LayoutElement> Tile<W> {
         });
     }
 
+    pub fn offset_move_y_anim_current(&mut self, offset: f64) {
+        if let Some(move_) = self.move_y_animation.as_mut() {
+            // If the anim is almost done, there's little point trying to offset it; we can let
+            // things jump. If it turns out like a bad idea, we could restart the anim intead.
+            let value = move_.anim.value();
+            if value > 0.001 {
+                move_.from += offset / value;
+            }
+        }
+    }
+
     pub fn stop_move_animations(&mut self) {
         self.move_x_animation = None;
         self.move_y_animation = None;