fix(tui): calm legacy Windows console rendering (#1655)

CHANGELOG.md

@@ -43,6 +43,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   `composer_arrows_scroll` now defaults on for Windows terminals even when
   mouse capture is enabled, so wheel events that arrive as arrow keys scroll the
   transcript instead of cycling composer history (#1578).
+- **Plain Windows PowerShell / ConHost uses calmer rendering.** Unmarked
+  legacy Windows console hosts now automatically enable low-motion rendering,
+  disable fancy animations, and resolve `synchronized_output = "auto"` to off
+  so streaming redraws do not overlap or visibly flicker (#1590).
 
 ### Thanks
 
@@ -53,7 +57,9 @@ switching fixes harvested from #1642. Thanks to **Photo
 ([@eng2007](https://github.com/eng2007))** for the provider-aware `/model`
 picker catalog work harvested from #1201. Thanks to
 **[@kunpeng-ai-lab](https://github.com/kunpeng-ai-lab)** for the Windows
-composer scroll fix harvested from #1578.
+composer scroll fix harvested from #1578, and **WuMing
+([@asdfg314284230](https://github.com/asdfg314284230))** for the Windows
+PowerShell flicker fix harvested from #1591.
 
 ## [0.8.37] - 2026-05-14
 

crates/tui/CHANGELOG.md

@@ -43,6 +43,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   `composer_arrows_scroll` now defaults on for Windows terminals even when
   mouse capture is enabled, so wheel events that arrive as arrow keys scroll the
   transcript instead of cycling composer history (#1578).
+- **Plain Windows PowerShell / ConHost uses calmer rendering.** Unmarked
+  legacy Windows console hosts now automatically enable low-motion rendering,
+  disable fancy animations, and resolve `synchronized_output = "auto"` to off
+  so streaming redraws do not overlap or visibly flicker (#1590).
 
 ### Thanks
 
@@ -53,7 +57,9 @@ switching fixes harvested from #1642. Thanks to **Photo
 ([@eng2007](https://github.com/eng2007))** for the provider-aware `/model`
 picker catalog work harvested from #1201. Thanks to
 **[@kunpeng-ai-lab](https://github.com/kunpeng-ai-lab)** for the Windows
-composer scroll fix harvested from #1578.
+composer scroll fix harvested from #1578, and **WuMing
+([@asdfg314284230](https://github.com/asdfg314284230))** for the Windows
+PowerShell flicker fix harvested from #1591.
 
 ## [0.8.37] - 2026-05-14
 

crates/tui/src/main.rs

@@ -2398,6 +2398,13 @@ async fn run_doctor(config: &Config, workspace: &Path, config_path_override: Opt
         );
         any_quirk = true;
     }
+    if crate::settings::detected_legacy_windows_console_host() {
+        println!(
+            "  {} legacy Windows console host → low_motion + fancy_animations=false + synchronized_output=off (auto)",
+            "•".truecolor(sky_r, sky_g, sky_b)
+        );
+        any_quirk = true;
+    }
     if !any_quirk {
         println!(
             "  {} no env-driven terminal-quirk overrides active",

crates/tui/src/settings.rs

@@ -411,6 +411,18 @@ impl Settings {
             self.fancy_animations = false;
         }
 
+        // Plain Windows PowerShell / cmd.exe under legacy ConHost exposes none
+        // of the modern terminal markers below. Keep rendering calmer there:
+        // lower the motion rate, disable animated chrome, and avoid DEC 2026
+        // synchronized-output wrapping unless the user explicitly forced it on.
+        if detected_legacy_windows_console_host() {
+            self.low_motion = true;
+            self.fancy_animations = false;
+            if self.synchronized_output.eq_ignore_ascii_case("auto") {
+                self.synchronized_output = "off".to_string();
+            }
+        }
+
         // Ptyxis 50.x (the new default terminal on Ubuntu 26.04) ships with
         // VTE 0.84.x which mishandles DEC mode 2026 synchronized output: the
         // begin/end pair is parsed but each wrapped frame still triggers a
@@ -907,6 +919,31 @@ pub fn detected_ptyxis_terminal() -> bool {
     matches!(std::env::var("PTYXIS_VERSION"), Ok(v) if !v.trim().is_empty())
 }
 
+/// Returns `true` for the unmarked Windows console-host path used by plain
+/// PowerShell / cmd.exe. Modern Windows terminals set at least one marker that
+/// lets us keep the richer rendering path.
+pub fn detected_legacy_windows_console_host() -> bool {
+    cfg!(windows)
+        && legacy_windows_console_host_env([
+            std::env::var_os("WT_SESSION").as_deref(),
+            std::env::var_os("ConEmuPID").as_deref(),
+            std::env::var_os("TERM_PROGRAM").as_deref(),
+            std::env::var_os("WEZTERM_EXECUTABLE").as_deref(),
+            std::env::var_os("WEZTERM_PANE").as_deref(),
+            std::env::var_os("ALACRITTY_WINDOW_ID").as_deref(),
+            std::env::var_os("ANSICON").as_deref(),
+            std::env::var_os("TERM").as_deref(),
+        ])
+}
+
+fn legacy_windows_console_host_env(markers: [Option<&std::ffi::OsStr>; 8]) -> bool {
+    fn has_value(value: Option<&std::ffi::OsStr>) -> bool {
+        value.is_some_and(|v| !v.is_empty())
+    }
+
+    markers.into_iter().all(|value| !has_value(value))
+}
+
 fn normalize_optional_background_color(value: Option<&str>) -> Option<String> {
     value.and_then(|raw| normalize_background_color_setting(raw).ok().flatten())
 }
@@ -1200,6 +1237,14 @@ mod tests {
     #[test]
     fn no_animations_env_recognises_truthy_spellings_only() {
         let _g = no_animations_test_guard();
+        let prev_wt_session = std::env::var_os("WT_SESSION");
+        // The test is about NO_ANIMATIONS only. On Windows CI, an unmarked
+        // console host now independently enables low_motion, so mark the host
+        // as non-legacy while checking falsy spellings.
+        #[cfg(windows)]
+        unsafe {
+            std::env::set_var("WT_SESSION", "test");
+        }
         for truthy in ["1", "true", "True", "YES", "on"] {
             // SAFETY: serialised by the guard.
             unsafe {
@@ -1221,6 +1266,10 @@ mod tests {
         // SAFETY: cleanup under the guard.
         unsafe {
             std::env::remove_var("NO_ANIMATIONS");
+            match prev_wt_session {
+                Some(v) => std::env::set_var("WT_SESSION", v),
+                None => std::env::remove_var("WT_SESSION"),
+            }
         }
     }
 
@@ -1417,6 +1466,93 @@ mod tests {
         }
     }
 
+    #[test]
+    fn legacy_windows_console_host_detects_unmarked_shell() {
+        assert!(legacy_windows_console_host_env([
+            None, None, None, None, None, None, None, None
+        ]));
+    }
+
+    #[test]
+    fn legacy_windows_console_host_excludes_modern_terminal_markers() {
+        use std::ffi::OsStr;
+
+        let marker = Some(OsStr::new("1"));
+        assert!(!legacy_windows_console_host_env([
+            marker, None, None, None, None, None, None, None
+        ]));
+        assert!(!legacy_windows_console_host_env([
+            None, marker, None, None, None, None, None, None
+        ]));
+        assert!(!legacy_windows_console_host_env([
+            None, None, marker, None, None, None, None, None
+        ]));
+        assert!(!legacy_windows_console_host_env([
+            None, None, None, marker, None, None, None, None
+        ]));
+        assert!(!legacy_windows_console_host_env([
+            None, None, None, None, marker, None, None, None
+        ]));
+        assert!(!legacy_windows_console_host_env([
+            None, None, None, None, None, marker, None, None
+        ]));
+        assert!(!legacy_windows_console_host_env([
+            None, None, None, None, None, None, marker, None
+        ]));
+        assert!(!legacy_windows_console_host_env([
+            None, None, None, None, None, None, None, marker
+        ]));
+    }
+
+    #[cfg(windows)]
+    #[test]
+    fn unmarked_windows_console_forces_calm_rendering() {
+        let _g = term_program_test_guard();
+        let vars = [
+            "WT_SESSION",
+            "ConEmuPID",
+            "TERM_PROGRAM",
+            "WEZTERM_EXECUTABLE",
+            "WEZTERM_PANE",
+            "ALACRITTY_WINDOW_ID",
+            "ANSICON",
+            "TERM",
+            "SSH_CLIENT",
+            "SSH_TTY",
+            "NO_ANIMATIONS",
+            "PTYXIS_VERSION",
+        ];
+        let prev: Vec<_> = vars
+            .iter()
+            .map(|name| (*name, std::env::var_os(name)))
+            .collect();
+
+        // SAFETY: serialised by the guard.
+        unsafe {
+            for name in vars {
+                std::env::remove_var(name);
+            }
+        }
+
+        let mut settings = Settings::default();
+        assert!(!settings.low_motion, "default is animated");
+        assert_eq!(settings.synchronized_output, "auto");
+        settings.apply_env_overrides();
+        assert!(settings.low_motion);
+        assert!(!settings.fancy_animations);
+        assert_eq!(settings.synchronized_output, "off");
+
+        // SAFETY: cleanup under the guard.
+        unsafe {
+            for (name, value) in prev {
+                match value {
+                    Some(value) => std::env::set_var(name, value),
+                    None => std::env::remove_var(name),
+                }
+            }
+        }
+    }
+
     #[test]
     fn ssh_session_forces_low_motion_on() {
         let _g = term_program_test_guard();

crates/tui/src/tui/ui.rs

@@ -327,16 +327,16 @@ pub async fn run_tui(config: &Config, options: TuiOptions) -> Result<()> {
     let backend = ColorCompatBackend::new(stdout, color_depth, palette_mode);
     let mut terminal = Terminal::new(backend)?;
     // At this point Settings hasn't loaded yet, so we can't read the
-    // user's `synchronized_output` knob. Use the same env-based Ptyxis
-    // detection that `Settings::apply_env_overrides` uses, so the
+    // user's `synchronized_output` knob. Use the same env-based terminal
+    // quirk detection that `Settings::apply_env_overrides` uses, so the
     // startup viewport reset matches what every later draw will do on
-    // this terminal. A user who has explicitly set
-    // `synchronized_output = "on"` to override Ptyxis detection will
-    // get sync wrap from the main draw loop onward; the one-time
-    // startup viewport reset stays opt-out for them, which is the safe
-    // default because the cost is at most brief tearing on the first
-    // frame.
-    let sync_output_at_init = !crate::settings::detected_ptyxis_terminal();
+    // flicker-sensitive hosts. A user who has explicitly set
+    // `synchronized_output = "on"` to override detection will get sync wrap
+    // from the main draw loop onward; the one-time startup viewport reset
+    // stays opt-out for them, which is the safe default because the cost is
+    // at most brief tearing on the first frame.
+    let sync_output_at_init = !crate::settings::detected_ptyxis_terminal()
+        && !crate::settings::detected_legacy_windows_console_host();
     reset_terminal_viewport(&mut terminal, sync_output_at_init)?;
     let event_broker = EventBroker::new();