Blocking alt keys on macOS

[chrisduerr]

In a terminal emulator like Alacritty, it's possible to send control characters directly to the shell using the alt and control keys. However this gets problematic with keyboard layouts that use the alt key for accessing alternative key characters. This is mainly an issue on macOS since other platforms usually use the AltGr key.

The concrete problem is that when alt is held down, the modified key is received, instead of the normal key. So for example Option + F will generate ƒ instead of f. In iTerm it's possible to pick between sending escapes and sending modified keys, however Alacritty currently can't offer that since winit will always send the modified keys.

Now I'm not really familiar with how this works under the hood in winit, but ideally I'd like an option in the window builder (maybe possible to change at runtime?) to prevent alt from modifying the characters, so that Option + F will still send f.

Alternatively it would be necessary for Alacritty to define its own mappings and ignore the ReceivedCharacter event, which would likely result in a bit of duplication between winit and Alacritty.

More information about this issue can be found here:
alacritty/alacritty#2017

[eraserhd]

Also alacritty/alacritty#1610

[kjmph]

I caught this sight of this bug from Alacritty. See my misguided post from alacritty/alacritty#2017. I opened a pull request #1436. However, that is meant to be an example pull request, as I'm not a maintainer of winit, I'm not as familiar with how options work.

[kjmph]

If anyone would like to test, the pull request went through a few rounds. I have an open pull request that fixes this issue:

#1449

If anyone can test to provide support for the pull request, there is a single change to alacritty:

diff --git a/alacritty/src/window.rs b/alacritty/src/window.rs
index 71327ba..7bc555a 100644
--- a/alacritty/src/window.rs
+++ b/alacritty/src/window.rs
@@ -265,7 +265,8 @@ impl Window {
             .with_title(title)
             .with_visible(false)
             .with_transparent(true)
-            .with_maximized(window_config.startup_mode() == StartupMode::Maximized);
+            .with_maximized(window_config.startup_mode() == StartupMode::Maximized)
+            .with_ignore_alt_modifier(true);

         match window_config.decorations {
             Decorations::Full => window,

[kchibisov]

If someone is going to test it on alacritty they could apply the following patch to use mentioned PR in addition to the patch above.

diff --git a/Cargo.toml b/Cargo.toml
index 415ceb9..e0db73b 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -13,3 +13,4 @@ incremental = false

 [patch.crates-io]
 servo-freetype-sys = { path = "servo-freetype-proxy" }
+winit = { git = "https://github.com/kjmph/winit", branch = "fix-alt-modifier" }

[jkp]

If someone is going to test it on alacritty they could apply the following patch to use mentioned PR in addition to the patch above.

diff --git a/Cargo.toml b/Cargo.toml
index 415ceb9..e0db73b 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -13,3 +13,4 @@ incremental = false

 [patch.crates-io]
 servo-freetype-sys = { path = "servo-freetype-proxy" }
+winit = { git = "https://github.com/kjmph/winit", branch = "fix-alt-modifier" }

What is this suppose to do? I built and tested and hoped it would allow me to map left-alt to escape as in terminal.app but that doesn't seem to be the case...what am I missing?

[kjmph]

Can you elaborate on your difficulty? I believe what is wrong is that you are attempting to build an un-modified Alacritty against this branch of Winit. However, this patch requires modifications to Alacritty after it went through review. It now defaults the window setting to false and Alacritty must enable it. I haven't opened that pull request for Alacritty, since the upstream Winit pull request needs to be merged first. Would you like me to open it so you can see if the changes work for you?

To clarify a bit as well, this patch treats Option as Alt. So, either Left-Option or Right-Option will result in the Alt modifier set on the raw key press event. Alacritty interprets Alt as Escape, and will do as you are suggesting, which is to send an escape code like Terminal.app. Without this patch, the diacritical mark will still be sent, resulting in oddities for the next key press event. See this bug report for more information:

alacritty/alacritty#2017