Allow using slider velocity control in toolbox to adjust velocity of selection (#38106)

https://github.com/user-attachments/assets/8d5f94b2-d7b6-4418-a8c2-2fb85aa77bb3

---

RFC

I have received information from @peppy that in the course of
preparations for the next lazer changelog video, @pishifat has expressed
dissatisfaction with the operation of the slider velocity control added
in #37746. If I understood correctly, the
issue was that the slider velocity control in question did not attempt
to interact with the current selection of objects. @peppy has requested
me to attempt to fix it before the next lazer release.

This PR contains an implementation of allowing the slider velocity
control to adjust the velocity of all selected sliders, if any.

I am not particularly happy with the resulting code for this feature,
but I also do not have many better ideas. Despite my attempts at
streamlining the logic after the initial pass, the implementation is
still tricky because it attempts to do much magic, both in trying to
achieve unobtrusive operation and sufficiently informative visual state.

There are subtle details that may turn out to be unsatisfactory to
mappers in practice. For one instance, the following flow may be of
concern:

1. Start with no selection. In this state the velocity control follows
the last slider by default.
2. Choose a velocity value manually.
3. Select an object.
4. Deselect the object.
5. The velocity value chosen in (2) is dropped.

Such behaviours can be brought back if desired, but at cost of further
additional complexity of the logic of the feature.

Author: bdach

osu.Game.Rulesets.Osu.Tests/Editor/TestSceneSliderVelocityAdjust.cs

@@ -7,7 +7,6 @@
 using osu.Framework.Testing;
 using osu.Framework.Utils;
 using osu.Game.Beatmaps;
-using osu.Game.Graphics.UserInterface;
 using osu.Game.Rulesets.Edit;
 using osu.Game.Rulesets.Osu.Edit;
 using osu.Game.Rulesets.Osu.Objects;
@@ -137,15 +136,15 @@ public void TestVelocityUndo()
         [Test]
         public void TestVelocityToolbox()
         {
-            ExpandableSlider<double> velocitySlider = null!;
+            OsuSliderVelocityToolboxGroup.ExpandableSliderVelocityControl velocitySlider = null!;
             ExpandableButton useLastSliderButton = null!;
 
             AddStep("enter editor", () => Game.ScreenStack.Push(new EditorLoader()));
             AddUntilStep("wait for editor load", () => editor?.ReadyForUse == true);
             AddStep("retrieve controls", () =>
             {
                 var toolbox = this.ChildrenOfType<OsuSliderVelocityToolboxGroup>().Single();
-                velocitySlider = toolbox.ChildrenOfType<ExpandableSlider<double>>().Single();
+                velocitySlider = toolbox.ChildrenOfType<OsuSliderVelocityToolboxGroup.ExpandableSliderVelocityControl>().Single();
                 useLastSliderButton = toolbox.ChildrenOfType<ExpandableButton>().Single();
             });
 
@@ -193,6 +192,25 @@ public void TestVelocityToolbox()
             AddUntilStep("wait for expand", () => useLastSliderButton.Expanded.Value, () => Is.True);
             AddAssert("use last slider button disabled", () => useLastSliderButton.Enabled.Value, () => Is.False);
 
+            AddStep("select fourth slider", () => editorBeatmap.SelectedHitObjects.Add(editorBeatmap.HitObjects[3]));
+            AddAssert("velocity slider at 3x", () => velocitySlider.Current.Value, () => Is.EqualTo(3));
+            AddStep("set 1.5x velocity", () => velocitySlider.Current.Value = 1.5);
+            AddAssert("fourth slider has 1.5x velocity", () => ((Slider)editorBeatmap.HitObjects[3]).SliderVelocityMultiplier, () => Is.EqualTo(1.5));
+
+            AddStep("select first and last slider", () =>
+            {
+                editorBeatmap.SelectedHitObjects.Clear();
+                editorBeatmap.SelectedHitObjects.Add(editorBeatmap.HitObjects[0]);
+                editorBeatmap.SelectedHitObjects.Add(editorBeatmap.HitObjects[^1]);
+            });
+            AddAssert("velocity slider at 1x", () => velocitySlider.Current.Value, () => Is.EqualTo(1));
+            AddStep("set 5x velocity", () => velocitySlider.Current.Value = 5);
+            AddAssert("first slider has 5x velocity", () => ((Slider)editorBeatmap.HitObjects[0]).SliderVelocityMultiplier, () => Is.EqualTo(5));
+            AddAssert("last slider has 5x velocity", () => ((Slider)editorBeatmap.HitObjects[^1]).SliderVelocityMultiplier, () => Is.EqualTo(5));
+
+            AddStep("clear selection", () => editorBeatmap.SelectedHitObjects.Clear());
+            AddAssert("velocity slider at 2x", () => velocitySlider.Current.Value, () => Is.EqualTo(2));
+
             void placeSlider()
             {
                 AddStep("enter slider placement mode", () => InputManager.Key(Key.Number3));