From d6a51776e56b2da5b5da22f1ca6abed912dcf3f0 Mon Sep 17 00:00:00 2001
From: Ellpeck <me@ellpeck.de>
Date: Sat, 24 Sep 2022 11:04:23 +0200
Subject: [PATCH] Fixed the scroll bar of an empty panel being positioned
 incorrectly

---
 CHANGELOG.md              |  1 +
 MLEM.Ui/Elements/Panel.cs | 37 ++++++++++++++++++++-----------------
 2 files changed, 21 insertions(+), 17 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1a9434d..22fc6bd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -46,6 +46,7 @@ Fixes
 - Fixed UiMarkdownParser not parsing formatting in headings and blockquotes
 - Fixed Element.OnChildAdded and Element.OnChildRemoved being called for grandchildren when a child is added
 - Fixed an exception when trying to force-update the area of an element without a ui system
+- Fixed the scroll bar of an empty panel being positioned incorrectly
 
 ### MLEM.Data
 Additions
diff --git a/MLEM.Ui/Elements/Panel.cs b/MLEM.Ui/Elements/Panel.cs
index 4be979a..6ccf880 100644
--- a/MLEM.Ui/Elements/Panel.cs
+++ b/MLEM.Ui/Elements/Panel.cs
@@ -84,8 +84,9 @@ namespace MLEM.Ui.Elements {
                         return;
                     if (e == null || !e.GetParentTree().Contains(this))
                         return;
-                    var firstChild = this.Children.First(c => c != this.ScrollBar);
-                    this.ScrollBar.CurrentValue = (e.Area.Center.Y - this.Area.Height / 2 - firstChild.Area.Top) / e.Scale + this.ChildPadding.Value.Height / 2;
+                    var firstChild = this.Children.FirstOrDefault(c => c != this.ScrollBar);
+                    if (firstChild != null)
+                        this.ScrollBar.CurrentValue = (e.Area.Center.Y - this.Area.Height / 2 - firstChild.Area.Top) / e.Scale + this.ChildPadding.Value.Height / 2;
                 };
                 this.AddChild(this.ScrollBar);
             }
@@ -231,28 +232,30 @@ namespace MLEM.Ui.Elements {
         protected virtual void ScrollSetup() {
             if (!this.scrollOverflow || this.IsHidden)
                 return;
-            // if there is only one child, then we have just the scroll bar
-            if (this.Children.Count == 1)
-                return;
 
-            // the "real" first child is the scroll bar, which we want to ignore
-            var firstChild = this.Children.First(c => c != this.ScrollBar);
-            var lowestChild = this.GetLowestChild(c => c != this.ScrollBar && !c.IsHidden);
-            var childrenHeight = lowestChild.Area.Bottom - firstChild.Area.Top;
+            float childrenHeight;
+            if (this.Children.Count > 1) {
+                var firstChild = this.Children.FirstOrDefault(c => c != this.ScrollBar);
+                var lowestChild = this.GetLowestChild(c => c != this.ScrollBar && !c.IsHidden);
+                childrenHeight = lowestChild.Area.Bottom - firstChild.Area.Top;
+            } else {
+                // if we only have one child (the scroll bar), then the children take up no visual height
+                childrenHeight = 0;
+            }
 
-            // the max value of the scrollbar is the amount of non-scaled pixels taken up by overflowing components
+            // the max value of the scroll bar is the amount of non-scaled pixels taken up by overflowing components
             var scrollBarMax = (childrenHeight - this.ChildPaddedArea.Height) / this.Scale;
             if (!this.ScrollBar.MaxValue.Equals(scrollBarMax, Element.Epsilon)) {
                 this.ScrollBar.MaxValue = scrollBarMax;
                 this.relevantChildrenDirty = true;
+            }
 
-                // update child padding based on whether the scroll bar is visible
-                var childOffset = this.ScrollBar.IsHidden ? 0 : this.ScrollerSize.Value.X + this.ScrollBarOffset;
-                if (!this.scrollBarChildOffset.Equals(childOffset, Element.Epsilon)) {
-                    this.ChildPadding += new Padding(0, -this.scrollBarChildOffset + childOffset, 0, 0);
-                    this.scrollBarChildOffset = childOffset;
-                    this.SetAreaDirty();
-                }
+            // update child padding based on whether the scroll bar is visible
+            var childOffset = this.ScrollBar.IsHidden ? 0 : this.ScrollerSize.Value.X + this.ScrollBarOffset;
+            if (!this.scrollBarChildOffset.Equals(childOffset, Element.Epsilon)) {
+                this.ChildPadding += new Padding(0, -this.scrollBarChildOffset + childOffset, 0, 0);
+                this.scrollBarChildOffset = childOffset;
+                this.SetAreaDirty();
             }
 
             // the scroller height has the same relation to the scroll bar height as the visible area has to the total height of the panel's content