`Viewfinder.visibleGameSize` is not reapplied when the viewport changes size

What happened?

The zoom that visibleGameSize sets depends on the size of the viewport, but it's only recalculated in Viewfinder.onGameResize (viewfinder.dart#L221-L224), not when the size of the viewport itself changes. This breaks in three ways:

1. The viewport is resized by the game. When a FixedSizeViewport is resized at runtime, for example to expand a minimap from 256×160 to 576×352, the zoom stays at 0.44. The level then only fills the middle of the expanded minimap, with 1296×792 units visible instead of 576×352.
2. The viewport of a camera that the game added itself is replaced. camera.viewport = ... adds the new viewport after the viewfinder (camera_component.dart#L112-L117), so on a game resize, the viewfinder recalculates the zoom from the old size of the viewport, and only then is the viewport resized. The zoom is one resize behind from then on: after maximizing the window it's still the zoom for the old size, and after restoring it, it's the zoom for the maximized window. FlameGame.camera isn't affected, since FlameGame.onGameResize resizes the viewport of that camera first (flame_game.dart#L219), but every other camera is, like the cameras added for split-screen or minimaps.
3. visibleGameSize is set before the viewport has a size. For example CameraComponent(world: world)..viewfinder.visibleGameSize = Vector2(100, 100) before the camera is added to the game sets the zoom to 0, which throws zoom level must be positive: 0.0.

https://github.com/user-attachments/assets/72c6672a-39cc-4102-9304-4f15c1b29479

https://github.com/user-attachments/assets/e6adbf48-3354-4ae7-89c0-bc7320124d1f

What do you expect?

The zoom is recalculated whenever the size of the viewport changes, so the region given to visibleGameSize always fits into it, including when the viewport only gets its size later.

How can we reproduce this?

1. Add a second camera with a FixedSizeViewport(256, 160), and set its visibleGameSize to the size of the level, 576×352. The zoom is 0.44.
2. Set minimap.viewport.size = Vector2(576, 352).
3. The zoom stays at 0.44 instead of 1.0.

What steps should take to fix this?

The viewport calls Viewfinder.onViewportResize whenever its size changes (viewport.dart#L88). That will recalculate the zoom, which covers all three cases and makes the onGameResize override unnecessary. _updateZoom will also skip the update while the viewport doesn't have a size yet, instead of setting the zoom to 0.

Do have an example of where the bug occurs?

import 'dart:ui';

import 'package:flame/camera.dart';
import 'package:flame/components.dart';
import 'package:flame/game.dart';
import 'package:flutter/material.dart' show Colors;
import 'package:flutter/widgets.dart';

void main() => runApp(GameWidget(game: ReproGame()));

class ReproGame extends FlameGame {
  @override
  Future<void> onLoad() async {
    final levelSize = Vector2(576, 352);
    world.add(
      RectangleComponent(
        size: levelSize,
        paint: Paint()..color = Colors.brown,
      ),
    );

    final minimap = CameraComponent(
      world: world,
      viewport: FixedSizeViewport(256, 160)..position = Vector2.all(16),
    );
    minimap.viewfinder.position = levelSize / 2;
    add(minimap);
    minimap.viewfinder.visibleGameSize = levelSize;

    // Expand the minimap after two seconds.
    add(
      TimerComponent(
        period: 2,
        removeOnFinish: true,
        onTick: () => minimap.viewport.size = Vector2(576, 352),
      ),
    );
  }
}

Expected: after two seconds, the level fills the expanded minimap. Actual: the zoom stays at 0.44, and the level stays small in the middle of the minimap.

For the second case, add the camera to the game yourself, replace its viewport, and then resize the window:

class ReproGame extends FlameGame {
  @override
  Future<void> onLoad() async {
    final arena = World()
      ..add(
        RectangleComponent(
          size: Vector2(330, 196),
          anchor: Anchor.center,
          paint: Paint()..color = Colors.brown,
        ),
      );
    final camera = CameraComponent(
      world: arena,
      viewport: FixedAspectRatioViewport(aspectRatio: 16 / 9),
    );
    addAll([arena, camera]);
    camera.viewfinder.visibleGameSize = Vector2(330, 196);

    // Switch to filling the window, like from a display mode setting.
    add(
      TimerComponent(
        period: 2,
        removeOnFinish: true,
        onTick: () => camera.viewport = MaxViewport(),
      ),
    );
  }
}

Expected: after the switch, the arena fits the window at every window size. Actual: after maximizing the window the arena is too small, and after restoring it the arena is cut off.

Relevant log output

# Only for the third case, otherwise nothing is logged or thrown.
'package:flame/src/camera/viewfinder.dart': Failed assertion: line 61 pos 12: 'value > 0': zoom level must be positive: 0.0

Execute in a terminal and put output into the code block below

[√] Flutter (Channel stable, 3.47.9, on Microsoft Windows [Version 10.0.22631.6199], locale en-US)
    • Flutter version 3.47.9 on channel stable
    • Framework revision c197ca5199 (4 days ago), 2026-10-29 10:10:20 -0400
    • Engine revision 89c39edfdf
    • Dart version 3.13.7
    • DevTools version 2.60.0

[√] Windows Version (11 Pro 64-bit, 23H2, 2009)
[√] Android toolchain - develop for Android devices (Android SDK version 36.1.0)
[√] Chrome - develop for the web
[√] Visual Studio - develop Windows apps (Visual Studio Community 2026 18.11.2)
[√] Connected device (3 available)
[√] Network resources

• No issues found!

Affected platforms

All

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