Fix SplitView layout during window resizing under AppKitBackend

The layout went a bit bonkers during window resizing before this fix. When the resize
required the panes to resize they just wouldn't and the split view would end up bigger
than the window and flicker back and forth randomly as you continued resizing.

Author: stackotter

Sources/AppKitBackend/AppKitBackend.swift

@@ -653,6 +653,7 @@ class NSSplitViewResizingDelegate: NSObject, NSSplitViewDelegate {
     var leadingWidth = 0
     var minimumLeadingWidth = 0
     var maximumLeadingWidth = 0
+    var isFirstUpdate = true
 
     func setResizeHandler(_ handler: @escaping (_ paneWidths: [Int]) -> Void) {
         resizeHandler = handler
@@ -693,7 +694,15 @@ class NSSplitViewResizingDelegate: NSObject, NSSplitViewDelegate {
 
     func splitView(_ splitView: NSSplitView, resizeSubviewsWithOldSize oldSize: NSSize) {
         splitView.adjustSubviews()
-        splitView.setPosition(200, ofDividerAt: 0)
+
+        if isFirstUpdate {
+            splitView.setPosition(max(200, CGFloat(minimumLeadingWidth)), ofDividerAt: 0)
+            isFirstUpdate = false
+        } else {
+            // Magic! Thanks https://stackoverflow.com/a/30494691. This one line fixed all
+            // of the split view resizing issues.
+            splitView.setPosition(splitView.subviews[0].frame.width, ofDividerAt: 0)
+        }
     }
 }