Text2d scalefactor change detection fix

[ickshonpe]

Objective

Text2d doesn't respond to changes to the window scalefactor.

Fixes #16223

Solution

In update_text2d_layout store the previous scale factor in a Local instead and check against the current scale factor to detect changes.

It seems like previously the text wasn't updated because of a bug with the WindowScaleFactorChanged event and it isn't emitted after changes to the scale factor. That needs to be looked into, but this will work for now.

Testing

Really simple app that draws a big message in the middle of the window:

use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .run();
}

fn setup(mut commands: Commands) {
    commands.spawn(Camera2d);
    commands.spawn((
        Text2d::new("Hello"),
        TextFont {
            font_size: 400.,
            ..Default::default()
        },
    ));
}

Looks fine:

On main, after changing the monitor's scale factor:

With this PR the text maintains the same size and position after the scale factor is changed.

[mockersf]

the WindowScaleFactorChanged event is also used to update ui layout, I would prefer a complete fix rather than "just" text.

The issue is probably about the first scale change on window creation. I can't reproduce on macOS, but it may be os dependent

[ickshonpe]

the WindowScaleFactorChanged event is also used to update ui layout, I would prefer a complete fix rather than "just" text.

It's actually more complicated than just fixing the event, there are a couple of issues with update_text2d_layout:

• It regenerates the text2d layout on recieving a WindowScaleFactorChanged event but the render target could be a different window or a texture.
• It uses the scale factor from the primary window regardless of the render target.
• It doesn't delete the old unused font atlases for the previous scale factors.

So storing the scale factor in a local variable is marginally preferable to watching the event. With the event (if it worked) it triggers updates on changes to the scale factor of any window, not just the primary window. But since text2d always uses the scale factor of the primary window, the text reupdate is needless on changes to non-primary window's scalefactors.

It's stupid how it works this way though and we need proper multi-window support.

The issue is probably about the first scale change on window creation. I can't reproduce on macOS, but it may be os dependent

It can be tricky sometimes to capture scale factor bugs in examples. Easiest way in this case is if you setup two monitors with different scale factors and drag the bevy app from one to the other.