Fix text selection when dragging window with hidden titlebar

[liby]

When macos-titlebar-style is set to hidden, dragging the window near the
titlebar area would inadvertently trigger text selection. This was caused
by the mouseDown event being passed to the surface even when in the
titlebar region.

The fix improves the logic in mouseDown handler to properly handle window
dragging without triggering text selection when clicking in the titlebar
area. This provides a smoother window dragging experience while preserving
expected text selection behavior in the terminal area.

demo.mov

Fixes #5259

[mitchellh]

One comment. I want to think about this approach a bit more and just explore if there's any other way to detect a titlebar area click but this seems agreeable otherwise.

[mitchellh]

I don't think there's any reason to cache this. It seems fairly cheap to calculate on the fly, and that removes a whole lot of complexity around keeping it up to date during various events that I suspect we could get wrong at some point.

[liby]

I agree that caching the titlebar height isn't necessary. The calculation is very lightweight and removing the cache eliminates the complexity of keeping it in sync across various window events.

Regarding alternative approaches for detecting titlebar clicks, I've thought about it and here are some considerations:

• Using hitTest might be less reliable since the titlebar area is part of our view when using hidden titlebar style

• Adding a separate view for the titlebar would add complexity without clear benefits

• Using window delegate methods would be too late as we need to intercept the click before window dragging

I'm open to exploring other approaches if you have specific ideas in mind!