window (mac): recalculate sizes on new tab

When a new tab is added, the tab bar may appear which causes our size
grid size calculation to become invalidated. We weren't updating this
previously, which had the effect of mouse selection being in the wrong
place. There were likely other effects...

Fixes https://github.com/mitchellh/ghostty/issues/69

src/Window.zig

@@ -492,7 +492,7 @@ pub fn shouldClose(self: Window) bool {
 }
 
 /// Add a window to the tab group of this window.
-pub fn addWindow(self: Window, other: *Window) void {
+pub fn addWindow(self: *Window, other: *Window) void {
     assert(builtin.target.isDarwin());
 
     // This has a hard dependency on GLFW currently. If we want to support
@@ -507,6 +507,19 @@ pub fn addWindow(self: Window, other: *Window) void {
         objc.Object.fromId(other_win),
         NSWindowOrderingMode.above,
     });
+
+    // Adding a new tab can cause the tab bar to appear which changes
+    // our viewport size. We need to call the size callback in order to
+    // update values. For example, we need this to set the proper mouse selection
+    // point in the grid.
+    const size = self.window.getSize() catch |err| {
+        log.err("error querying window size for size callback on new tab err={}", .{err});
+        return;
+    };
+    self.sizeCallback(size) catch |err| {
+        log.err("error in size callback from new tab err={}", .{err});
+        return;
+    };
 }
 
 /// Called from the app thread to handle mailbox messages to our specific