SidePanel::rights with an exact_width have incorrect padding

[a99984b1799]

Describe the bug

SidePanel::right incorrectly pads the right side of the panel when it has an exact_width set.

To Reproduce

The following update function roughly implements the screenshot below:

fn update(&mut self, ctx: &egui::Context, _: &mut eframe::Frame) {
    egui::SidePanel::right("right").exact_width(100.).show(ctx, |ui| {
        ui.label("AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA");
    });
    egui::SidePanel::left("left").exact_width(100.).show(ctx, |ui| {
        ui.label("AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA");
    });
    egui::CentralPanel::default().show(ctx, |ui| { });
}

Screenshots

Desktop (please complete the following information):

• OS: Linux.

Additional Information

As implied, the padding is correct when an exact_width is not given.

[a99984b1799]

Closely related to #3006, I wouldn't call it a duplicate but I'd be happy to close this if need be.

[RylanYancey]

I am also experiencing this problem! Here is what I'm seeing:

The 'saved moves' panel is being rendered on top of 'Nxf6' and the multiline text widget. Heres' my src:

(screenshot cuz my editor is being weird about copying)

I'm calculating what I want the width to be (40% of the screen space for the notes panel, and 10% for the moves panel), then assigning it by using exact_width(). However, this results in the aforementioned issue.

Thanks!
- Rylan Yancey