macos: make terminal smaller to account for legacy scrollbar

When the preferred scrollbar style is "legacy", the scrollbar takes up
space that offsets the actual terminal. To prevent reflow, we detect
this before the scrollbar becomes visible and shrink our terminal width
to prepare for it.

This doesn't account for the style changing at runtime, yet.

Author: mitchellh

macos/Sources/Ghostty/SurfaceScrollView.swift

@@ -142,18 +142,35 @@ class SurfaceScrollView: NSView {
         // Only update sizes if we have a valid (non-zero) content size. The content size
         // can be zero when this is added early to a view, or to an invisible hierarchy.
         // Practically, this happened in the quick terminal.
-        let contentSize = scrollView.contentSize
-        if contentSize.width > 0 && contentSize.height > 0 {
-            // Keep document width synchronized with content width
-            documentView.setFrameSize(CGSize(
-                width: contentSize.width,
-                height: documentView.frame.height
-            ))
-            
-            // Inform the actual pty of our size change
-            surfaceView.sizeDidChange(contentSize)
+        var contentSize = scrollView.contentSize
+        guard contentSize.width > 0 && contentSize.height > 0 else {
+            synchronizeSurfaceView()
+            return
         }
 
+        // If we have a legacy scrollbar and its not visible, then we account for that
+        // in advance, because legacy scrollbars change our contentSize and force reflow
+        // of our terminal which is not desirable.
+        // See: https://github.com/ghostty-org/ghostty/discussions/9254
+        let style = scrollView.verticalScroller?.scrollerStyle ?? NSScroller.preferredScrollerStyle
+        if style == .legacy {
+            if (scrollView.verticalScroller?.isHidden ?? true) {
+                let scrollerWidth = NSScroller.scrollerWidth(for: .regular, scrollerStyle: .legacy)
+                contentSize.width -= scrollerWidth
+            }
+        }
+        
+        // Keep document width synchronized with content width
+        documentView.setFrameSize(CGSize(
+            width: contentSize.width,
+            height: documentView.frame.height
+        ))
+        
+        // Inform the actual pty of our size change. This doesn't change the actual view
+        // frame because we do want to render the whole thing, but it will prevent our
+        // rows/cols from going into the non-content area.
+        surfaceView.sizeDidChange(contentSize)
+        
         // When our scrollview changes make sure our surface view is synchronized
         synchronizeSurfaceView()
     }