Split out the base design of sheared overlay into its own abstract class

This will allow for reuse with the first-run overlay.

osu.Game/Overlays/Mods/ShearedOverlayContainer.cs

@@ -0,0 +1,136 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using osu.Framework.Allocation;
+using osu.Framework.Graphics;
+using osu.Framework.Graphics.Containers;
+using osu.Framework.Graphics.Shapes;
+using osu.Game.Graphics.Containers;
+using osu.Game.Graphics.UserInterface;
+
+namespace osu.Game.Overlays.Mods
+{
+    /// <summary>
+    /// A sheared overlay which provides a header and footer and basic animations.
+    /// Exposes <see cref="TopLevelContent"/>, <see cref="MainAreaContent"/> and <see cref="Footer"/> as valid targets for content.
+    /// </summary>
+    public abstract class ShearedOverlayContainer : OsuFocusedOverlayContainer
+    {
+        [Cached]
+        protected readonly OverlayColourProvider ColourProvider;
+
+        /// <summary>
+        /// The overlay's header.
+        /// </summary>
+        protected PopupScreenTitle Header { get; private set; }
+
+        /// <summary>
+        /// The overlay's footer.
+        /// </summary>
+        protected Container Footer { get; private set; }
+
+        /// <summary>
+        /// A container containing all content, including the header and footer.
+        /// May be used for overlay-wide animations.
+        /// </summary>
+        protected Container TopLevelContent { get; private set; }
+
+        /// <summary>
+        /// A container for content that is to be displayed between the header and footer.
+        /// </summary>
+        protected Container MainAreaContent { get; private set; }
+
+        /// <summary>
+        /// A container for content that is to be displayed inside the footer.
+        /// </summary>
+        protected Container FooterContent { get; private set; }
+
+        protected abstract OverlayColourScheme ColourScheme { get; }
+
+        protected override bool StartHidden => true;
+
+        protected override bool BlockNonPositionalInput => true;
+
+        protected ShearedOverlayContainer()
+        {
+            RelativeSizeAxes = Axes.Both;
+
+            ColourProvider = new OverlayColourProvider(ColourScheme);
+        }
+
+        [BackgroundDependencyLoader]
+        private void load()
+        {
+            const float footer_height = 50;
+
+            Child = TopLevelContent = new Container
+            {
+                RelativeSizeAxes = Axes.Both,
+                Children = new Drawable[]
+                {
+                    Header = new PopupScreenTitle
+                    {
+                        Anchor = Anchor.TopCentre,
+                        Depth = float.MinValue,
+                        Origin = Anchor.TopCentre,
+                        Title = "Mod Select",
+                        Description = "Mods provide different ways to enjoy gameplay. Some have an effect on the score you can achieve during ranked play. Others are just for fun.",
+                        Close = Hide
+                    },
+                    MainAreaContent = new Container
+                    {
+                        RelativeSizeAxes = Axes.Both,
+                        Padding = new MarginPadding
+                        {
+                            Top = PopupScreenTitle.HEIGHT,
+                            Bottom = footer_height,
+                        }
+                    },
+                    Footer = new Container
+                    {
+                        RelativeSizeAxes = Axes.X,
+                        Depth = float.MinValue,
+                        Height = footer_height,
+                        Margin = new MarginPadding { Top = 10 },
+                        Anchor = Anchor.BottomCentre,
+                        Origin = Anchor.BottomCentre,
+                        Children = new Drawable[]
+                        {
+                            new Box
+                            {
+                                RelativeSizeAxes = Axes.Both,
+                                Colour = ColourProvider.Background5
+                            },
+                            FooterContent = new Container
+                            {
+                                RelativeSizeAxes = Axes.Both,
+                            },
+                        }
+                    }
+                }
+            };
+        }
+
+        protected override void PopIn()
+        {
+            const double fade_in_duration = 400;
+
+            base.PopIn();
+            this.FadeIn(fade_in_duration, Easing.OutQuint);
+
+            Header.MoveToY(0, fade_in_duration, Easing.OutQuint);
+            Footer.MoveToY(0, fade_in_duration, Easing.OutQuint);
+        }
+
+        protected override void PopOut()
+        {
+            const double fade_out_duration = 500;
+
+            base.PopOut();
+            this.FadeOut(fade_out_duration, Easing.OutQuint);
+
+            Header.MoveToY(-Header.DrawHeight, fade_out_duration, Easing.OutQuint);
+            Footer.MoveToY(Footer.DrawHeight, fade_out_duration, Easing.OutQuint);
+        }
+    }
+}