macOS: Remove hidden titlebar safe area and pad document height in SurfaceScrollView (#9241)

Fixes #9233. The issue was that the content view was effectively
truncated by `SurfaceScrollView`s safe area insets covering the titlebar
(I suppose the scroll view doesn't inherit the `ignoresSafeArea`
modifier on `TerminalSplitTreeView` because it's not actually a node in
that tree, it just contains a subview that is). I think it's OK to zero
these insets unconditionally, as I don't think the frame size passed
down from `SurfaceWrapper` will ever overlap with a visible titlebar.

This PR also fixes a somewhat related issue I discovered along the way.
In many cases, the mouse wouldn't work on the first row of text, whether
trying to select text or interact with TUI elements like neovim
bufferlines/tablines. When looking at the inspector while moving the
mouse around, you'd see the mouse position jump discontinuously from a
y-coordinate of 10-20 pixels to -3, i.e., no longer within the surface.

The problem turned out to be that the height of the document view only
included the text lines, not the window padding, while the content view
is sized to the surface including padding. If you imagine the
metaphorical sliding of the viewport up and down the document, and you
require the text to snap to a fixed grid on the viewport, it's clear
that the document must have the same top and bottom padding as the
viewport, otherwise the math breaks at the ends. Sure enough, adding
this padding fixed the problem.

To properly reproduce these issues and see the effect of the fixes, it's
helpful to change the system settings to always show scroll bars
(Appearance -> Show scroll bars -> Always). That's what connecting an
external mouse was all about. You should also remove the debug banner or
use a release build, otherwise the banner takes the place of the hidden
titlebar and conceals the issue.

macos/Sources/Ghostty/SurfaceScrollView.swift

@@ -119,6 +119,11 @@ class SurfaceScrollView: NSView {
         observers.forEach { NotificationCenter.default.removeObserver($0) }
     }
 
+    // The entire bounds is a safe area, so we override any default
+    // insets. This is necessary for the content view to match the
+    // surface view if we have the "hidden" titlebar style.
+    override var safeAreaInsets: NSEdgeInsets { return NSEdgeInsetsZero }
+
     override func setFrameSize(_ newSize: NSSize) {
         super.setFrameSize(newSize)
         
@@ -222,10 +227,16 @@ class SurfaceScrollView: NSView {
         let cellHeight = surfaceView.cellSize.height
         guard cellHeight > 0 else { return }
 
+        // The full document height must include the vertical padding around the cell
+        // grid, otherwise the content view ends up misaligned with the surface.
+        let documentGridHeight = CGFloat(scrollbar.total) * cellHeight
+        let gridHeight = CGFloat(scrollbar.len) * cellHeight
+        let padding = scrollView.contentSize.height - gridHeight
+        let documentHeight = documentGridHeight + padding
+
         // Our width should be the content width to account for visible scrollers.
         // We don't do horizontal scrolling in terminals.
-        let totalHeight = CGFloat(scrollbar.total) * cellHeight
+        let newSize = CGSize(width: scrollView.contentSize.width, height: documentHeight)
-        let newSize = CGSize(width: scrollView.contentSize.width, height: totalHeight)
         documentView.setFrameSize(newSize)
         
         // Only update our actual scroll position if we're not actively scrolling.