perf: remove large drawing stalls

Keep resident geometry and GPU resources stable across edits, MSPACE zoom, and Model/Paper switches. Add unified PERF output for tracing remaining costs.

src/perf.rs

@@ -0,0 +1,62 @@
+//! Shared performance trace for terminal output and the in-app `PERF` panel.
+
+use std::collections::VecDeque;
+use std::fmt;
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::{Mutex, OnceLock};
+
+const MAX_LINES: usize = 500;
+
+static UI_ENABLED: AtomicBool = AtomicBool::new(false);
+static ENV_ENABLED: OnceLock<bool> = OnceLock::new();
+static LINES: OnceLock<Mutex<VecDeque<String>>> = OnceLock::new();
+
+fn lines() -> &'static Mutex<VecDeque<String>> {
+    LINES.get_or_init(|| Mutex::new(VecDeque::with_capacity(MAX_LINES)))
+}
+
+/// True when tracing was requested with `PERF=1` or the in-app panel is open.
+pub fn enabled() -> bool {
+    UI_ENABLED.load(Ordering::Relaxed)
+        || *ENV_ENABLED.get_or_init(|| std::env::var_os("PERF").is_some())
+}
+
+/// Enable or disable collection driven by the in-app `PERF` command.
+pub fn set_ui_enabled(enabled: bool) {
+    UI_ENABLED.store(enabled, Ordering::Relaxed);
+}
+
+/// Write one performance line to stderr and retain it for the in-app panel.
+pub fn record(args: fmt::Arguments<'_>) {
+    if !enabled() {
+        return;
+    }
+    let line = args.to_string();
+    eprintln!("{line}");
+    let mut entries = lines().lock().unwrap_or_else(|e| e.into_inner());
+    if entries.len() == MAX_LINES {
+        entries.pop_front();
+    }
+    entries.push_back(line);
+}
+
+/// Plain-text snapshot used by the panel and its Copy button.
+pub fn snapshot_text() -> String {
+    let entries = lines().lock().unwrap_or_else(|e| e.into_inner());
+    entries
+        .iter()
+        .map(String::as_str)
+        .collect::<Vec<_>>()
+        .join("\n")
+}
+
+pub fn clear() {
+    lines().lock().unwrap_or_else(|e| e.into_inner()).clear();
+}
+
+#[macro_export]
+macro_rules! perf_record {
+    ($($arg:tt)*) => {
+        $crate::perf::record(format_args!($($arg)*))
+    };
+}