Fix/truecolor windows 1305 (#1307)

* fix: restore TrueColor support on Windows Terminal (fix #1305)

* Fix terminal color support issues and add automated Windows VM testing script

- Honor the NO_COLOR environment variable.
- Fix color support detection on Apple's Terminal.app.
- Correctly treat empty terminal name / terminal emulator name as unidentified.
- Safely downgrade to Palette16 on legacy Windows consoles rejecting VT processing.
- Add tools/windows-vm.sh and tools/windows-vm/provision.ps1 to automate VM spin-up, provisioning, and testing.

Author: ArthurSonzogni

.gitignore

@@ -88,8 +88,11 @@ out/
 !tools/**/*.sh
 !tools/**/*.py
 !tools/**/*.cpp
+!tools/**/*.ps1
 !tools/abi_fingerprint.txt
 !tools/abidiff_suppressions.ini
+# Windows VM disk images and ssh keys, created by tools/windows-vm.sh:
+tools/windows-vm/data/
 build/
 build_asan/
 builddir/

CHANGELOG.md

@@ -4,13 +4,21 @@ Changelog
 Next
 ====
 
+### Screen
+- Bugfix: Restore TrueColor support on Windows Terminal (default to TrueColor on Windows and check `WT_SESSION` environment variable for WSL compatibility). See #1305.
+- Feature: Honor the `NO_COLOR` environment variable (https://no-color.org). When set and non-empty, colors degrade to the terminal's default colors.
+- Bugfix: Apple's Terminal.app (`TERM_PROGRAM=Apple_Terminal`) is now reported as `Palette256` instead of `TrueColor`; it does not support 24bit colors.
+- Bugfix: An empty terminal name or terminal emulator name is now treated as unidentified by `Terminal::ComputeColorSupport`, instead of implying TrueColor support.
+- Bugfix (Windows): Downgrade color support when the console rejects VT processing (legacy consoles), instead of emitting TrueColor escape sequences.
+
 ### Build
 - Bugfix: Fix build failure when an older FTXUI is installed in a system
   include path (e.g. MacPorts upgrade). A CMake deduplication quirk was
   promoting the project's own `-I include/` to `-isystem`, causing package
   managers' `-I/opt/local/include` (which may contain stale headers) to
   win. See #1299, #1300.
 
+
 7.0.0 (2026-06-13)
 ------------------
 

src/ftxui/screen/color.cpp

@@ -101,14 +101,22 @@ Color::Color(Palette1 /*value*/) : Color() {}
 
 /// @brief Build a color using the Palette16 colors.
 Color::Color(Palette16 index)
-    : type_(ColorType::Palette16), red_(index), alpha_(255) {}
+    : type_(ColorType::Palette16), red_(index), alpha_(255) {
+  if (Terminal::ColorSupport() == Terminal::Color::Palette1) {
+    type_ = ColorType::Palette1;
+  }
+}
 
 /// @brief Build a color using Palette256 colors.
 Color::Color(Palette256 index)
     : type_(ColorType::Palette256), red_(index), alpha_(255) {
   if (Terminal::ColorSupport() >= Terminal::Color::Palette256) {
     return;
   }
+  if (Terminal::ColorSupport() == Terminal::Color::Palette1) {
+    type_ = ColorType::Palette1;
+    return;
+  }
   type_ = ColorType::Palette16;
   red_ = GetColorInfo(Color::Palette256(red_)).index_16;
 }
@@ -129,6 +137,10 @@ Color::Color(uint8_t red, uint8_t green, uint8_t blue, uint8_t alpha)
   if (Terminal::ColorSupport() == Terminal::Color::TrueColor) {
     return;
   }
+  if (Terminal::ColorSupport() == Terminal::Color::Palette1) {
+    type_ = ColorType::Palette1;
+    return;
+  }
 
   // Find the closest Color from the database:
   const int max_distance = 256 * 256 * 3;

src/ftxui/screen/color_test.cpp

@@ -3,10 +3,45 @@
 // the LICENSE file.
 #include "ftxui/screen/color.hpp"
 #include <gtest/gtest.h>
+#include <cstdlib>  // for std::getenv, setenv, unsetenv
+#include <optional>
+#include <string>
 #include "ftxui/screen/terminal.hpp"
 
 namespace ftxui {
 
+#if !defined(_WIN32)
+namespace {
+// Clear an environment variable for the duration of a scope, restoring its
+// original value on destruction. ComputeColorSupport reads NO_COLOR and
+// WT_SESSION from the environment; tests must not depend on the machine
+// running them.
+class ScopedClearEnv {
+ public:
+  explicit ScopedClearEnv(const char* name) : name_(name) {
+    const char* value = std::getenv(name);  // NOLINT
+    if (value != nullptr) {
+      saved_ = value;
+    }
+    unsetenv(name);
+  }
+  ~ScopedClearEnv() {
+    if (saved_) {
+      setenv(name_, saved_->c_str(), 1);
+    } else {
+      unsetenv(name_);
+    }
+  }
+  ScopedClearEnv(const ScopedClearEnv&) = delete;
+  ScopedClearEnv& operator=(const ScopedClearEnv&) = delete;
+
+ private:
+  const char* name_;
+  std::optional<std::string> saved_;
+};
+}  // namespace
+#endif
+
 TEST(ColorTest, PrintTransparent) {
   Terminal::SetColorSupport(Terminal::Color::TrueColor);
   EXPECT_EQ(Color().Print(false), "39");
@@ -83,4 +118,86 @@ TEST(ColorTest, HSV) {
   EXPECT_EQ(Color::HSV(0, 255, 255).Print(false), "38;2;255;0;0");
 }
 
+TEST(ColorTest, ComputeColorSupport) {
+#if !defined(_WIN32)
+  const ScopedClearEnv no_color("NO_COLOR");
+  const ScopedClearEnv wt_session("WT_SESSION");
+
+  EXPECT_EQ(Terminal::ComputeColorSupport("xterm", "256", "", "", "", {}),
+            Terminal::Color::Palette256);
+  EXPECT_EQ(Terminal::ComputeColorSupport("xterm-256color", "", "", "", "", {}),
+            Terminal::Color::Palette256);
+
+  // An unidentified terminal without any hint only gets Palette16.
+  EXPECT_EQ(Terminal::ComputeColorSupport("", "", "", "unknown", "unknown", {}),
+            Terminal::Color::Palette16);
+  // An empty terminal name is treated as unidentified.
+  EXPECT_EQ(Terminal::ComputeColorSupport("", "", "", "", "", {}),
+            Terminal::Color::Palette16);
+
+  // Apple's Terminal.app supports 256 colors, but not 24bit ones.
+  EXPECT_EQ(Terminal::ComputeColorSupport("", "", "Apple_Terminal", "unknown",
+                                          "unknown", {}),
+            Terminal::Color::Palette256);
+  EXPECT_EQ(Terminal::ComputeColorSupport("", "", "iTerm.app", "unknown",
+                                          "unknown", {}),
+            Terminal::Color::TrueColor);
+#endif
+  EXPECT_EQ(Terminal::ComputeColorSupport("xterm", "truecolor", "", "", "", {}),
+            Terminal::Color::TrueColor);
+  EXPECT_EQ(Terminal::ComputeColorSupport("kitty", "", "", "", "", {}),
+            Terminal::Color::TrueColor);
+}
+
+#if !defined(_WIN32)
+TEST(ColorTest, ComputeColorSupportWTSession) {
+  const ScopedClearEnv no_color("NO_COLOR");
+  const ScopedClearEnv wt_session("WT_SESSION");
+
+  // Without WT_SESSION, an unidentified terminal only gets Palette16.
+  EXPECT_EQ(Terminal::ComputeColorSupport("", "", "", "unknown", "unknown", {}),
+            Terminal::Color::Palette16);
+
+  // Windows Terminal sets WT_SESSION, including when running under WSL.
+  setenv("WT_SESSION", "test_session_id", 1);
+  EXPECT_EQ(Terminal::ComputeColorSupport("", "", "", "unknown", "unknown", {}),
+            Terminal::Color::TrueColor);
+}
+
+TEST(ColorTest, ComputeColorSupportNoColor) {
+  const ScopedClearEnv no_color("NO_COLOR");
+  const ScopedClearEnv wt_session("WT_SESSION");
+
+  // NO_COLOR (https://no-color.org) disables colors, even on a TrueColor
+  // capable terminal.
+  setenv("NO_COLOR", "1", 1);
+  EXPECT_EQ(Terminal::ComputeColorSupport("xterm", "truecolor", "", "unknown",
+                                          "unknown", {}),
+            Terminal::Color::Palette1);
+
+  // NO_COLOR takes precedence over WT_SESSION.
+  setenv("WT_SESSION", "test_session_id", 1);
+  EXPECT_EQ(Terminal::ComputeColorSupport("", "", "", "unknown", "unknown", {}),
+            Terminal::Color::Palette1);
+
+  // An empty NO_COLOR is the same as an unset one.
+  setenv("NO_COLOR", "", 1);
+  unsetenv("WT_SESSION");
+  EXPECT_EQ(Terminal::ComputeColorSupport("xterm", "truecolor", "", "unknown",
+                                          "unknown", {}),
+            Terminal::Color::TrueColor);
+}
+#endif
+
+TEST(ColorTest, FallbackToPalette1) {
+  Terminal::SetColorSupport(Terminal::Color::Palette1);
+  // Every color degrades to the terminal's default colors.
+  EXPECT_EQ(Color(Color::Red).Print(false), "39");
+  EXPECT_EQ(Color(Color::Red).Print(true), "49");
+  EXPECT_EQ(Color(Color::DarkRed).Print(false), "39");
+  EXPECT_EQ(Color::RGB(1, 2, 3).Print(false), "39");
+  EXPECT_EQ(Color::RGB(1, 2, 3).Print(true), "49");
+  Terminal::SetColorSupport(Terminal::Color::TrueColor);
+}
+
 }  // namespace ftxui

src/ftxui/screen/screen.cpp

@@ -57,7 +57,12 @@ void WindowsEmulateVT100Terminal() {
   auto stdout_handle = GetStdHandle(STD_OUTPUT_HANDLE);
 
   DWORD out_mode = 0;
-  GetConsoleMode(stdout_handle, &out_mode);
+  if (!GetConsoleMode(stdout_handle, &out_mode)) {
+    // The output is not a console (e.g. redirected to a file or a pipe). Keep
+    // the detected color support and let the consumer of the stream interpret
+    // the escape sequences.
+    return;
+  }
 
   // https://docs.microsoft.com/en-us/windows/console/setconsolemode
   const int enable_virtual_terminal_processing = 0x0004;

src/ftxui/screen/terminal.cpp

@@ -209,12 +209,44 @@ Color ComputeColorSupport(std::string_view term,
 }
 
 Color TerminalInfo::ComputeColorSupport() const {
-  // 0. Platform specific overrides.
+  // TODO(v8): Read NO_COLOR and WT_SESSION from ComputeColorSupportInternal()
+  // and pass them in as parameters, so that this function remains a pure
+  // function of its inputs. This requires extending the public
+  // Terminal::ComputeColorSupport() signature, i.e. an API-breaking change.
+
+  // 0. User preference. See https://no-color.org.
+  const char* no_color = std::getenv("NO_COLOR");  // NOLINT
+  if (no_color != nullptr && no_color[0] != '\0') {
+    return Terminal::Color::Palette1;
+  }
+
+  // 1. Platform specific overrides.
 #if defined(__EMSCRIPTEN__)
   return Terminal::Color::TrueColor;
 #endif
+#if defined(_WIN32)
+  // Check if we are running in a console, and if that console supports VT processing.
+  auto stdout_handle = GetStdHandle(STD_OUTPUT_HANDLE);
+  DWORD out_mode = 0;
+  if (GetConsoleMode(stdout_handle, &out_mode)) {
+    const int enable_virtual_terminal_processing = 0x0004;
+    const int disable_newline_auto_return = 0x0008;
+    out_mode |= enable_virtual_terminal_processing;
+    out_mode |= disable_newline_auto_return;
+    if (!SetConsoleMode(stdout_handle, out_mode)) {
+      return Terminal::Color::Palette16;
+    }
+  }
+  return Terminal::Color::TrueColor;
+#endif
+
+  // Check WT_SESSION for Windows Terminal (e.g. when running under WSL).
+  const char* wt_session = std::getenv("WT_SESSION");  // NOLINT
+  if (wt_session != nullptr && wt_session[0] != '\0') {
+    return Terminal::Color::TrueColor;
+  }
 
-  // 1. term / colorterm environment variables.
+  // 2. term / colorterm environment variables.
   if (ContainsAny(impl_->colorterm, {"24bit", "truecolor"})) {
     return Terminal::Color::TrueColor;
   }
@@ -227,23 +259,27 @@ Color TerminalInfo::ComputeColorSupport() const {
     return Terminal::Color::Palette256;
   }
 
-  // 2. term_program
+  // 3. term_program
   if (ContainsAny(impl_->term_program, {
                                            "iterm",
-                                           "apple_terminal",
                                            "vscode",
                                            "warp",
                                            "ghostty",
                                            "wezterm",
                                        })) {
     return Terminal::Color::TrueColor;
   }
-  if (Contains(impl_->term_program, "iterm")) {
+  // Apple's Terminal.app (TERM_PROGRAM=Apple_Terminal) supports 256 colors,
+  // but not 24bit ones.
+  if (Contains(impl_->term_program, "apple_terminal")) {
     return Terminal::Color::Palette256;
   }
 
-  // 3. terminal identification.
-  if (impl_->terminal_emulator_name != "unknown") {
+  // 4. terminal identification.
+  // An empty name means the terminal was not identified, the same as
+  // "unknown".
+  if (!impl_->terminal_emulator_name.empty() &&
+      impl_->terminal_emulator_name != "unknown") {
     return Terminal::Color::TrueColor;
   }
   if (impl_->terminal_name == "xterm") {