Phantom key presses when focusing an egui/winit window

[hut]

Currently, when running egui in X11 on Linux with the winit backend, phantom key presses are registered when the window gains focus. Examples:

1. If you press Alt+TAB to select the window, a key press of "TAB" is erroneously registered.
2. My window manager has the key binding Alt+w to change workspaces. Assuming the egui window has a text box selected, if I change the workspace to the one with the egui window with the key binding Alt+w, it will erroneously type a "w" into the text box.

In either example, I would expect no key to be registered. After all, no other (non-broken) application will register/type keys simply by changing focus into the application window.

This error happens because winit sends "fake" KeyboardInput events with a field "is_synthetic" when the window focus is gained. Key presses with "is_synthetic == true" should not be interpreted as real key presses and should be filtered out, according to the maintainers of winit.

egui currently does not do special handling of KeyboardInput events with "is_synthetic == true", see

egui/crates/egui-winit/src/lib.rs

Line 371 in 8321f64

WindowEvent::KeyboardInput { event, .. } => {

The proper handling would be to discard the event if "keyboardinput.event.state == ElementState::Pressed && keyboardinput.is_synthetic" is true.

See also:

• Relevant winit bug report: Keys leak through on window focus change (alt+tab) rust-windowing/winit#3691
• Relevant winit discussion: Remove synthetic key events? rust-windowing/winit#3543
• An analogous fix in another UI framework: fix: ignore synthetic keyboard input lapce/floem#387
  • They discard all events with "is_synthetic == true", which is another way to handle this.

Thanks for looking into this. It probably causes a lot of confusion/frustration for X11 users that heavily customize their window manager key bindings.

[emilk]

Sounds like an easy fix then! Wanna make a PR?

[hut]

Sure, will do.

[hut]

Thanks so much for resolving this.

It's such a subtle bug, but so annoying. I'm using a tiling window manager with lots of alt+letter keys for focus switching, and all those unintended key presses are driving me crazy. Very relieved, thanks. <3