fix: normalize invalid RGB in keyframes

e2e/test_keyframe.py

@@ -39,7 +39,8 @@
 # marker; this is the "base paint" a real TUI does once on startup.
 BASE_PAINT = (
     "\x1b[?1049h\x1b[2J\x1b[H"
-    "\x1b[1;1H+==== DASHBOARD ====+"
+    "\x1b[1;1H\x1b[38;2;137;180;250;48;2;24;24;37m"
+    "+==== DASHBOARD ====+\x1b[0m"
     "\x1b[2;1H| loading...        |"
     "\x1b[3;1H+===================+"
 )
@@ -121,6 +122,13 @@ def test_late_joiner_sees_static_chrome_of_long_running_tui(
             "history was not evicted - raise NUM_UPDATES "
             f"(snapshot head: {snapshot[:80]!r})"
         )
+        assert "38;2;137;180;250" in snapshot, (
+            "the keyframe lost the base paint's truecolor"
+        )
+        assert "38:2:137:180:250" not in snapshot, (
+            "the keyframe kept avt's ambiguous RGB form, which xterm.js "
+            "reads shifted by one channel"
+        )
 
         # Behavior under test: a freshly-loaded viewer renders the static
         # chrome, not just the changing cell.

src/cli/screen.rs

@@ -90,7 +90,8 @@ impl Keyframer {
         }
         // avt uses the 8-bit CSI (U+009B); rewrite to the 7-bit `ESC [` form
         // that every terminal and xterm.js accept.
-        Some(dump.replace('\u{9b}', "\x1b[").into_bytes())
+        let dump = dump.replace('\u{9b}', "\x1b[");
+        Some(semicolon_sgr_params(&dump).into_bytes())
     }
 
     fn apply_resize(&mut self, size: TermSize) {
@@ -149,3 +150,37 @@ impl Keyframer {
         }
     }
 }
+
+/// avt writes truecolor as `38:2:R:G:B`, which T.416 does not allow: the
+/// color-space field is positional, not optional, so a reader following the
+/// spec takes G and B for R and G and defaults B to 0 - blue arrives as
+/// yellow-green. xterm.js follows the spec (xtermjs/xterm.js#5792 was closed
+/// as working-as-intended), so the emitter is what has to change.
+///
+/// The only colons avt puts inside an SGR are colors, and screen text can
+/// never contain an ESC, so rewriting every colon in a `...m` sequence is
+/// enough. A dump's trailing sequence can be unterminated - `Vt::dump`
+/// reproduces in-flight parser state - so the end of the dump terminates one
+/// too. A param that already spells the empty color space (`38:2::R:G:B`) is
+/// left alone: that form is valid, and flattening its `::` to `;;` would
+/// reintroduce the very shift this removes.
+fn semicolon_sgr_params(dump: &str) -> String {
+    let mut out = String::with_capacity(dump.len());
+    let mut rest = dump;
+    while let Some(start) = rest.find("\x1b[") {
+        let (head, body) = rest.split_at(start + 2);
+        out.push_str(head);
+        let end = body
+            .find(|c: char| !matches!(c, '0'..='9' | ';' | ':'))
+            .unwrap_or(body.len());
+        let (params, tail) = body.split_at(end);
+        if (tail.starts_with('m') || tail.is_empty()) && !params.contains("::") {
+            out.push_str(&params.replace(':', ";"));
+        } else {
+            out.push_str(params);
+        }
+        rest = tail;
+    }
+    out.push_str(rest);
+    out
+}