Button example is broken

[nicopap]

• Bevy version: 425570a (main as of today)

What you did

• Run the button example with cargo run --example button.

What went wrong

• At first, the window is transparent.
• When moving the mouse, it displays the button
• When hovering over the button, the button says "hover"
• When pressing and holding down the mouse left button, it keeps saying "hover"
• It only says "pressed" after releasing the mouse left button
• If I move afterward the mouse, it goes back to "hover"

What I expected

• Window shows up immediately
• Button should say "pressed" as soon as I start holding down the button
• Button should stop saying "pressed" when I stop holding down the button

Currently it's basically the inverse.

Additional information

Removing WinitSettings::desktop_app() fixes it.

This might be related to #11052. Supposedly, the updating mechanisms do not work as expected anymore.

[daxpedda]

Digging into this I can see that Bevy doesn't call Window::request_redraw() when using UpdateMode::Reactive when needed.
Currently it only calls it when:

• the user sends the RequestRedraw event
• when DeviceEvent::MouseMotion is received

This probably worked before because EventLoopWindowTarget::listen_device_events() was supported with very few targets.

So the solution is to call Window::request_redraw() when actually required instead of relying on circumstantial events.

I also noticed that Winit has set up a bool, WinitAppRunnerState::redraw_requested, to collect and call Window::request_redraw() later. This is unnecessary, Winit de-duplicates these requests already.

[daxpedda]

• At first, the window is transparent.

This is because SurfaceTexture::present() is not called on the first call to App::update(). I noticed that the whole render_system is not called for some reason.

If Bevy isn't ready to draw when RedrawRequested is sent, it has to schedule a new one to render when it is. Otherwise nothing will happen.

• When pressing and holding down the mouse left button, it keeps saying "hover"
• It only says "pressed" after releasing the mouse left button
• If I move afterward the mouse, it goes back to "hover"

If you move the mouse while holding it down it will actually update and show "pressed". This is because changes made to the button are only rendered in the next update cycle, not in the one they are changed in.
Keep in mind I'm just describing what I'm observing, I don't actually know why any of this is the way it is.

I'm not sure exactly how the rendering system in Bevy works, but sending RequestRedraw in Interaction::Pressed fixes that issue.

So either Bevy has to correctly render the current state in the same call to App::update(), or it has to schedule another redraw after changes are made.

[mockersf]

• When pressing and holding down the mouse left button, it keeps saying "hover"
• It only says "pressed" after releasing the mouse left button
• If I move afterward the mouse, it goes back to "hover"

system extract_uinodes is executed after system queue_uinodes, so a given frame is rendered with the nodes from the previous frame. This means that UI rendering is always one frame late