From 6d717c44fcfba44c8253c18fbfe657db594abe14 Mon Sep 17 00:00:00 2001
From: "fizzy-macos[bot]" <347071198+fizzy-macos[bot]@users.noreply.github.com>
Date: Mon, 26 Oct 2026 09:32:47 -0500
Subject: [PATCH] mobile: draw a coast at the fractional offset

Part of fizzyedit/work#362

Agent-Team: macos
Worker: claude

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
---
 core/widgets/ScrollArea.zig | 44 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 43 insertions(+), 1 deletion(-)

diff --git a/core/widgets/ScrollArea.zig b/core/widgets/ScrollArea.zig
index 54c8c7f15..6bd078b5a 100644
--- a/core/widgets/ScrollArea.zig
+++ b/core/widgets/ScrollArea.zig
@@ -27,6 +27,8 @@ const ScrollArea = @This();
 area: *dvui.ScrollAreaWidget,
 si: *dvui.ScrollInfo,
 touch: *Touch,
+/// dvui's `snap_to_pixels` as the area found it, put back by `deinit`.
+snap: bool,
 
 /// Per-area state that outlives a frame: a fling per axis and whether a finger is dragging.
 const Touch = struct {
@@ -58,8 +60,14 @@ pub fn init(src: std.builtin.SourceLocation, init_opts: dvui.ScrollAreaWidget.In
         .area = area,
         .si = area.si,
         .touch = dvui.dataGetPtrDefault(null, c.data().id, "_touch", Touch, .{}),
+        .snap = dvui.snapToPixels(),
     };
 
+    // A coast's last stretch moves a fraction of a pixel a frame: drawn at whole pixels it
+    // steps instead of gliding. Content is drawn at the fractional offset until the coast
+    // ends, and `deinit` snaps it again so text is crisp at rest.
+    if (self.touch.x.coasting or self.touch.y.coasting) _ = dvui.snapToPixelsSet(false);
+
     // A press while coasting only stops the coast: eat it so it doesn't also tap what is under it.
     for (dvui.events()) |*e| {
         if (!dvui.eventMatch(e, .{ .id = c.data().id, .r = c.data().borderRectScale().r })) continue;
@@ -125,6 +133,7 @@ pub fn deinit(self: *ScrollArea) void {
     // `processMotionScroll`; clear it so dvui's coast never runs beside ours.
     self.si.velocity = .{};
     self.area.deinit();
+    _ = dvui.snapToPixelsSet(self.snap);
 }
 
 /// Scroll by one touch motion, as dvui's `processMotionScroll` does but without setting
@@ -200,6 +209,10 @@ fn scrollBy(self: *ScrollArea, by: dvui.Point) void {
 
 var t_viewport_y: f32 = 0;
 var t_use_dvui: bool = false;
+/// Where a child of the area is drawn this frame (physical y, as the renderer would place it),
+/// and whether the renderer snaps there.
+var t_drawn_y: f32 = 0;
+var t_snapping: bool = false;
 
 fn testFrame() !dvui.App.Result {
     var root = dvui.box(@src(), .{}, .{ .expand = .both });
@@ -215,7 +228,10 @@ fn testFrame() !dvui.App.Result {
         const si = scroll.si;
         defer t_viewport_y = si.viewport.y;
         defer scroll.deinit();
-        _ = dvui.spacer(@src(), .{ .min_size_content = .{ .w = 100, .h = 20000 } });
+        const child = dvui.spacer(@src(), .{ .min_size_content = .{ .w = 100, .h = 20000 } });
+        t_snapping = dvui.snapToPixels();
+        const y = child.rectScale().r.y;
+        t_drawn_y = if (t_snapping) @round(y) else y;
     }
     return .ok;
 }
@@ -281,6 +297,32 @@ test "the coast is a function of time, not of how the frames cut it" {
     for (steady, jittery) |a, b| try std.testing.expectApproxEqAbs(a, b, 0.01);
 }
 
+test "a slow coast draws at a new place every frame, then snaps at rest" {
+    var t = try dvui.testing.init(.{ .window_size = .{ .w = 300, .h = 400 } });
+    defer t.deinit();
+    t_use_dvui = false;
+    _ = try swipe();
+    // 120 fps until the coast is slow: under 60 points a second is under half a point a frame.
+    var prev: f32 = t_drawn_y;
+    var slow_frames: usize = 0;
+    var last = t_viewport_y;
+    for (0..600) |_| {
+        try frameAfter(8);
+        const speed = (t_viewport_y - last) / 0.008;
+        last = t_viewport_y;
+        if (t_snapping) break;
+        if (speed < 60 and speed > 25) {
+            slow_frames += 1;
+            try std.testing.expect(t_drawn_y != prev);
+        }
+        prev = t_drawn_y;
+    }
+    try std.testing.expect(slow_frames > 20);
+    // At rest the area snaps again.
+    for (0..30) |_| try frameAfter(8);
+    try std.testing.expect(t_snapping);
+}
+
 test "a fling coasts at the speed of the last 100 ms, not of the last move" {
     var t = try dvui.testing.init(.{ .window_size = .{ .w = 300, .h = 400 } });
     defer t.deinit();