WindowSizePolicy::Content leaves a gap on Windows

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Building from `git+https://github.com/linebender/druid.git?branch=master#b6d389dec1aaafb239e7f0e72caac0d9de4285b6`.

For code,

```rust
use druid::widget::{TextBox};
use druid::{AppLauncher, Data, Lens, PlatformError, Widget, WidgetExt, WindowDesc};

#[derive(Clone, Data, Lens)]
struct State {
input: String,
}

fn main() -> Result<(), PlatformError> {
let main_window = WindowDesc::new(ui_builder())
.show_titlebar(true)
.window_size_policy(druid::WindowSizePolicy::Content);
let initial_state = State { input: String::new() };
AppLauncher::with_window(main_window)
.launch(initial_state)
}

fn ui_builder() -> impl Widget {
TextBox::new()
.lens(State::input)
.fix_width(350.0)
}
```

When `show_titlebar` is true, the main window size is perfect: https://snipboard.io/d2U4pv.jpg

When `show_titlebar` is false, the main window size is too tall, leaving a gap between the text box and the bottom border of the main window: https://snipboard.io/1o9ts4.jpg

This bug only happens on Windows. On Macos, both cases are fine.

Windows version info: 20H2 (OS Build 19042.1288)

---

Additional info: height of the gap changes with font family. For example, after I changed textbox's font to the monospace family, the gap got visibly bigger.

Reproduce the issue with the provided Rust example using WindowSizePolicy::Content, comparing show_titlebar(true) and false on Windows. Search the WindowSizePolicy implementation and Windows window-sizing entry points, then trace how the content height is calculated. Done means titlebar and borderless windows fit the TextBox without a gap, including with different font families.

Written by the indexing model from the issue text.