Fix terminal input reordering and Linux terminal rendering

Two separate defects behind the same report: typing in a container terminal
is sluggish on Linux, and a backspace can land *after* the characters typed
behind it.

The web terminal was the control that separated them. It shares the Docker
exec, the PTY, `exec_manager`, the input channel and its serial writer task,
and xterm.js itself — and it does not exhibit either symptom. Only three
things differ, and each accounts for part of the report.

**Input ordering.** Every keystroke was its own `invoke("terminal_input")`.
That command is `async`, so Tauri spawns each one as an independent task, and
those tasks then race for the session mutex in `ExecSessionManager::send_input`
— nothing preserved the order the bytes were typed in. The serial writer
downstream cannot help, because the order is already lost before anything
reaches the channel. The web terminal gets ordering for free by awaiting
`send_input` inline in a single WebSocket reader loop.

`useTerminal` now holds a per-session queue: one write in flight at a time,
the next only after the previous resolves. Anything typed meanwhile coalesces
into the next chunk, which also collapses a burst of typing into a couple of
IPC round trips rather than one per key. The queue is module scope, not hook
scope, because `useTerminal()` is called from several components — a per-hook
queue would leave speech-to-text, image paste and typing racing each other.
Each caller's promise still settles only when its own bytes have gone, so
`await sendInput(...)` keeps its meaning.

**The DMA-BUF escape hatch did not exist.** `apply_webkit_wayland_workaround`
left any pre-set value alone, including `0`, on a stated assumption that
WebKitGTK reads the variable as a boolean. It reads presence, so
`WEBKIT_DISABLE_DMABUF_RENDERER=0` disabled DMA-BUF exactly like `=1`, and no
value a user could set got the accelerated path back. `0`/`false`/`no`/empty
now remove the variable, which is the only thing WebKitGTK reads as enabled.
The default is unchanged: unset still means disabled on Linux.

**WebGL does not degrade to canvas here.** The comment on that workaround
assumed `@xterm/addon-webgl` would fall back to the canvas renderer once
DMA-BUF was off. Its constructor throws only when WebGL is *absent*, and with
DMA-BUF disabled WebGL is still present — served by software rasterisation.
So the addon loads and every frame is rendered on the CPU, slower than the
canvas renderer it was assumed to fall back to. `AppSettings::terminal_gpu_
rendering` decides whether it loads at all: `None` is auto (on for macOS and
Windows, off on Linux), `Some(_)` forces it either way from Settings →
Terminal. `Option<bool>` rather than `bool` so the zero value means "we
choose" instead of pinning every existing settings file to one answer.

Verified: 643 frontend tests and 530 Rust tests pass, clippy clean, secret
scan clean. The Linux rendering half needs confirming on a real desktop —
neither symptom reproduces in a headless container.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ApLYH6ybHwQFkMCtKuHrrV

app/src-tauri/src/main.rs

@@ -26,12 +26,27 @@
 /// their own init time, which happens inside the Tauri builder that
 /// function calls into, not at binary load.
 ///
-/// A user who has already set this themselves is left alone. That includes
-/// setting it to `0`, on the assumption WebKitGTK treats it as a boolean
-/// rather than presence-only — not verified against WebKitGTK's own source,
-/// so if it turns out to be presence-only, `=0` still reads as "set" here
-/// and disables DMA-BUF the same as any other value, which is at least the
-/// safe direction to be wrong in.
+/// A user who has already set this themselves is left alone — with one
+/// correction. The earlier version of this function left *any* pre-set value
+/// alone, including `0`, on the assumption WebKitGTK reads the variable as a
+/// boolean. WebKitGTK reads it as presence-only, so `WEBKIT_DISABLE_DMABUF_
+/// RENDERER=0` disabled DMA-BUF exactly like `=1` did, and there was no value
+/// at all a user could set to get the accelerated path back: the escape hatch
+/// the comment described did not exist. `0`, `false` and empty are now treated
+/// as an explicit opt-out and the variable is *removed*, which is the only
+/// thing WebKitGTK reads as "enabled". The default is unchanged — unset still
+/// means disabled on Linux, so nobody who was not deliberately overriding this
+/// sees any difference.
+///
+/// That matters more than it looks, because the trade described above is not
+/// the trade actually being made. `@xterm/addon-webgl` does not fall back to
+/// the canvas renderer here: its constructor throws only when WebGL is
+/// *absent*, and with DMA-BUF disabled WebGL is still present — served by
+/// software rasterisation. So the addon loads happily and every terminal frame
+/// is rendered on the CPU and copied, which is slower than the canvas renderer
+/// this comment assumed it would degrade to, not faster. See
+/// `terminal_gpu_rendering` in `AppSettings` for the switch that decides
+/// whether the addon is loaded at all.
 ///
 /// This env var also leaks to whatever the app spawns afterwards — notably
 /// a cold-launched default browser via the `opener` plugin's `xdg-open`
@@ -39,10 +54,77 @@
 /// URL; most non-WebKitGTK browsers ignore the variable entirely), but
 /// worth knowing before chasing the "links don't open" half of triple-c#34
 /// as a separate, unrelated cause.
+#[cfg(target_os = "linux")]
+const DMABUF_VAR: &str = "WEBKIT_DISABLE_DMABUF_RENDERER";
+
+/// What to do with `WEBKIT_DISABLE_DMABUF_RENDERER`, given whatever it is
+/// already set to. Split from the mutation so it can be tested without
+/// touching process-wide environment state from a parallel test runner.
+#[cfg(target_os = "linux")]
+#[derive(Debug, PartialEq, Eq)]
+enum DmabufAction {
+    /// Not set by the user — apply the workaround.
+    Disable,
+    /// Explicitly opted out. WebKitGTK reads presence, not value, so the only
+    /// way to express "enabled" is for the variable not to exist.
+    Remove,
+    /// Set to something meaning "disabled". Already what we want; leave it.
+    LeaveAlone,
+}
+
+#[cfg(target_os = "linux")]
+fn dmabuf_action(current: Option<&str>) -> DmabufAction {
+    match current {
+        None => DmabufAction::Disable,
+        Some(value) => match value.trim().to_ascii_lowercase().as_str() {
+            "" | "0" | "false" | "no" => DmabufAction::Remove,
+            _ => DmabufAction::LeaveAlone,
+        },
+    }
+}
+
 #[cfg(target_os = "linux")]
 fn apply_webkit_wayland_workaround() {
-    if std::env::var_os("WEBKIT_DISABLE_DMABUF_RENDERER").is_none() {
-        std::env::set_var("WEBKIT_DISABLE_DMABUF_RENDERER", "1");
+    let current = std::env::var(DMABUF_VAR).ok();
+    match dmabuf_action(current.as_deref()) {
+        DmabufAction::Disable => std::env::set_var(DMABUF_VAR, "1"),
+        DmabufAction::Remove => std::env::remove_var(DMABUF_VAR),
+        DmabufAction::LeaveAlone => {}
+    }
+}
+
+#[cfg(all(test, target_os = "linux"))]
+mod tests {
+    use super::{dmabuf_action, DmabufAction};
+
+    #[test]
+    fn unset_gets_the_workaround() {
+        assert_eq!(dmabuf_action(None), DmabufAction::Disable);
+    }
+
+    #[test]
+    fn falsey_values_opt_out_by_removing_the_variable() {
+        // The bug this replaces: these all previously read as "user set it,
+        // leave it alone", and WebKitGTK then disabled DMA-BUF anyway because
+        // it only checks presence. There was no way to ask for the GPU path.
+        for value in ["0", "false", "no", "", "  0  ", "FALSE", "No"] {
+            assert_eq!(
+                dmabuf_action(Some(value)),
+                DmabufAction::Remove,
+                "{value:?} should opt out"
+            );
+        }
+    }
+
+    #[test]
+    fn other_values_are_left_alone() {
+        for value in ["1", "true", "yes", "anything"] {
+            assert_eq!(
+                dmabuf_action(Some(value)),
+                DmabufAction::LeaveAlone,
+                "{value:?} should be left alone"
+            );
+        }
     }
 }