From 55a3ed38eaba0a7d73d69d1f9b8c60291824e0ac Mon Sep 17 00:00:00 2001
From: valadaptive <valadaptive@protonmail.com>
Date: Fri, 27 Sep 2024 08:04:52 -0400
Subject: [PATCH] Update ScrollArea drag velocity when drag stopped

Fixes #5174. The drag velocity was not being updated unless the cursor
counted as "dragging", which only happens when it's in motion. This
effectively guarantees that the drag velocity will never be zero, even
if the cursor is not moving, and results in spurious scroll velocity
being applied when the cursor is released. Instead, we update the
velocity only when the drag is stopped, which is when the kinetic
scrolling actually needs to begin.
---
 crates/egui/src/containers/scroll_area.rs | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/crates/egui/src/containers/scroll_area.rs b/crates/egui/src/containers/scroll_area.rs
index ab7da8aff29..b7da4bf8528 100644
--- a/crates/egui/src/containers/scroll_area.rs
+++ b/crates/egui/src/containers/scroll_area.rs
@@ -621,20 +621,35 @@ impl ScrollArea {
                 .interact_rect
                 .map(|rect| ui.interact(rect, id.with("area"), Sense::drag()));
 
-            if content_response_option.map(|response| response.dragged()) == Some(true) {
+            if content_response_option
+                .as_ref()
+                .is_some_and(|response| response.dragged())
+            {
                 for d in 0..2 {
                     if scroll_enabled[d] {
                         ui.input(|input| {
                             state.offset[d] -= input.pointer.delta()[d];
-                            state.vel[d] = input.pointer.velocity()[d];
                         });
                         state.scroll_stuck_to_end[d] = false;
                         state.offset_target[d] = None;
-                    } else {
-                        state.vel[d] = 0.0;
                     }
                 }
             } else {
+                // Apply the cursor velocity to the scroll area when the user releases the drag.
+                if content_response_option
+                    .as_ref()
+                    .is_some_and(|response| response.drag_stopped())
+                {
+                    for d in 0..2 {
+                        if scroll_enabled[d] {
+                            ui.input(|input| {
+                                state.vel[d] = input.pointer.velocity()[d];
+                            });
+                        } else {
+                            state.vel[d] = 0.0;
+                        }
+                    }
+                }
                 for d in 0..2 {
                     // Kinetic scrolling
                     let stop_speed = 20.0; // Pixels per second.