Anchored windows - first show draw misplacement

[zapp88]

When using .anchor on window - the first time the window is rendered results in window showing in "default" position for a brief moment and then jumping to proper anchor position. I belive this is caused by the way the anchor is implemented. We force the paint of the window since we dont know the size of its area - and as a result can't find where the "center" is. Maybe a sollution would be a "dry run" - that would result in size calculation but would not paint the window (resulting in flicker).

if let Some((anchor, offset)) = anchor {
    if is_new {
        // unknown size
        ctx.request_repaint();
    } else {
        let screen = ctx.available_rect();
        state.pos = anchor.align_size_within_rect(state.size, screen).min + offset;
    }
}

Video sample showing issue (the video is slowed down - since the effect last only a second):

Issue.mp4

Code to reproduce issue :

use eframe::{egui, emath::Align2};

fn main() {
    let options = eframe::NativeOptions::default();
    eframe::run_native(
        "Egui issue",
        options,
        Box::new(|_cc| Box::new(IssueApp { open: false })),
    );
}

struct IssueApp {
    open: bool,
}

impl eframe::App for IssueApp {
    fn update(&mut self, ctx: &egui::Context, _frame: &mut eframe::Frame) {
        egui::Window::new("Issue window")
            .anchor(Align2::CENTER_CENTER, [0., 0.])
            .open(&mut self.open)
            .show(ctx, |ui| {
                ui.label("Some content");
            });

        egui::CentralPanel::default().show(ctx, |ui| {
            if ui.button("Show window").clicked() {
                self.open = true;
            }
        });
    }
}

[zapp88]

Resolved via: #1856