fix: set max window clamp to current monitor size (#13171)

This PR fixes #7984. The issue was that GTK would clamp the window
itself based on the display it was opened on. We fix this by computing
the size based on the current display and then implicitly setting the
window size instead of relying on GTK to do it.

Claude Code w/ Opus 4.7 was used to investigate, fix and explain some of
the Ghostty architecture to me.

src/apprt/gtk/class/window.zig

@@ -1156,6 +1156,38 @@ pub const Window = extern struct {
         };
     }
 
+    fn toplevelComputeSize(
+        _: *gdk.Toplevel,
+        size: *gdk.ToplevelSize,
+        self: *Self,
+    ) callconv(.c) void {
+        // The compositor/quick terminal own the size in these states.
+        if (self.isMaximized() or
+            self.isFullscreen() or
+            self.isQuickTerminal()) return;
+
+        // If there's no GdkSurface yet these dimensions will be zero size which
+        // will make the window start out as small as possible. These checks ensure
+        // we don't start with a 0, 0 window size.
+        const gdk_surface = self.as(gtk.Native).getSurface() orelse return;
+        const w = gdk_surface.getWidth();
+        const h = gdk_surface.getHeight();
+        if (w <= 0 or h <= 0) return;
+
+        // GTK clamps the requested size to the compositor-reported bounds, which
+        // go stale when the window moves to a larger monitor. Only re-assert the
+        // current size when it exceeds those bounds; otherwise let GTK's default
+        // sizing win so a freshly-mapped window isn't forced to a tiny size.
+        var bounds_w: c_int = undefined;
+        var bounds_h: c_int = undefined;
+        size.getBounds(&bounds_w, &bounds_h);
+
+        if (bounds_w <= 0 or bounds_h <= 0) return;
+        if (w <= bounds_w and h <= bounds_h) return;
+
+        size.setSize(w, h);
+    }
+
     fn propFullscreened(
         _: *adw.ApplicationWindow,
         _: *gobject.ParamSpec,
@@ -1330,6 +1362,16 @@ pub const Window = extern struct {
                 self,
                 .{ .detail = "height" },
             );
+            // Connect after GTK's compute-size handler so our size wins.
+            if (gobject.ext.cast(gdk.Toplevel, gdk_surface)) |toplevel| {
+                _ = gdk.Toplevel.signals.compute_size.connect(
+                    toplevel,
+                    *Self,
+                    toplevelComputeSize,
+                    self,
+                    .{ .after = true },
+                );
+            }
         }
 
         // When we are realized we always setup our appearance since this