Check the canvas size before narrowing it to an int

A canvas side was cast to int with no range check, so a number that did
not fit came back as one that did: `-s 4294968296x600` rendered a
perfectly plausible 1000x600, and a hand-edited project file could ask
for a width the reader turned into something else (or, for a double out
of range, into undefined behaviour).

Both front-ends now check the side before the cast, against one shared
bound in chain.hpp — the 1..20000 the GUI spin buttons already offered,
so the help text, the CLI error, the project reader and the interface
all move together.

The tests gain a `canvas size` section covering the round trip at both
ends of the range and the refusals on either side of it, through the
project reader and through `-s`; the test binary picks up the CLI parser
to do it, which stays GUI-free.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/cli/options.cpp

@@ -24,7 +24,10 @@ bool parseSize(const std::string& text, int& width, int& height) {
     long w = 0;
     long h = 0;
     if (!parseLong(text.substr(0, sep), w) || !parseLong(text.substr(sep + 1), h)) return false;
-    if (w <= 0 || h <= 0) return false;
+    // Checked as longs, before the narrowing: 4294968296 cast straight to int
+    // is 1000, which would render a plausible canvas nobody asked for.
+    if (w < kMinCanvasSide || w > kMaxCanvasSide) return false;
+    if (h < kMinCanvasSide || h > kMaxCanvasSide) return false;
     width = static_cast<int>(w);
     height = static_cast<int>(h);
     return true;
@@ -66,7 +69,9 @@ Options parseOptions(int argc, char** argv) {
         } else if (arg == "-s" || arg == "--size") {
             const std::string value = requireValue(argc, argv, i, arg);
             if (!parseSize(value, options.width, options.height)) {
-                throw EffectError("invalid size '" + value + "', expected e.g. 1920x1080");
+                throw EffectError("invalid size '" + value + "', expected e.g. 1920x1080 with "
+                                  "each side between " + std::to_string(kMinCanvasSide) + " and " +
+                                  std::to_string(kMaxCanvasSide));
             }
         } else if (arg == "--seed") {
             const std::string value = requireValue(argc, argv, i, arg);
@@ -131,7 +136,8 @@ std::string usage(const char* programName) {
         << "Options:\n"
         << "  -e, --effect NAME     effect to apply, repeat to cascade (default: " << defaultEffectName() << ")\n"
         << "  -o, --output FILE     render headlessly to this file and exit\n"
-        << "  -s, --size WxH        canvas size (default: 1920x1080)\n"
+        << "  -s, --size WxH        canvas size, each side " << kMinCanvasSide << "-"
+        << kMaxCanvasSide << " (default: 1920x1080)\n"
         << "      --seed N          random seed (default: random)\n"
         << "      --list-effects    list available effects and exit\n"
         << "  -h, --help            show this help and exit\n";

src/core/chain.hpp

@@ -10,6 +10,13 @@
 
 namespace effector {
 
+// The canvas sides both front-ends accept. The upper bound is not a hardware
+// limit but the point past which a side stops fitting comfortably in an int
+// once multiplied out, so both the CLI and the project reader can reject a
+// number rather than silently wrap it into a plausible-looking one.
+inline constexpr int kMinCanvasSide = 1;
+inline constexpr int kMaxCanvasSide = 20000;
+
 // One link of a cascade: an effect together with the values it runs with.
 struct ChainStage {
     EffectPtr effect;

src/core/project.cpp

@@ -283,6 +283,21 @@ std::string relativeTo(const std::string& path, const std::string& directory) {
     return text.rfind("..", 0) == 0 ? path : text;
 }
 
+// A canvas side is checked as a double, before it is narrowed: casting a
+// number outside the range of an int is undefined, and even where it wraps
+// quietly it turns a size the file never named into one that looks ordinary.
+int canvasSide(const Json& value, const char* what) {
+    if (value.kind != Json::Kind::Number || value.number != std::trunc(value.number)) {
+        throw EffectError(std::string("the canvas ") + what + " must be a whole number");
+    }
+    if (!(value.number >= kMinCanvasSide) || !(value.number <= kMaxCanvasSide)) {
+        throw EffectError(std::string("the canvas ") + what + " must be between " +
+                          std::to_string(kMinCanvasSide) + " and " +
+                          std::to_string(kMaxCanvasSide) + " pixels");
+    }
+    return static_cast<int>(value.number);
+}
+
 std::string resolve(const std::string& path, const std::string& directory) {
     std::filesystem::path file(path);
     if (file.is_absolute() || directory.empty()) return path;
@@ -350,12 +365,11 @@ Project projectFromJson(const std::string& text, const std::string& directory) {
     Project project;
 
     if (const auto* canvas = root.member("canvas")) {
-        if (const auto* width = canvas->member("width")) project.width = static_cast<int>(width->number);
+        if (const auto* width = canvas->member("width")) project.width = canvasSide(*width, "width");
         if (const auto* height = canvas->member("height")) {
-            project.height = static_cast<int>(height->number);
+            project.height = canvasSide(*height, "height");
         }
     }
-    if (project.width <= 0 || project.height <= 0) throw EffectError("the canvas size is not positive");
 
     if (const auto* seed = root.member("seed")) {
         if (seed->kind == Json::Kind::Number) {

src/gui/window.cpp

@@ -176,10 +176,10 @@ Gtk::Widget& EffectorWindow::buildSidebar() {
     auto* grid = Gtk::make_managed<Gtk::Grid>();
     grid->set_column_spacing(8);
     grid->set_row_spacing(6);
-    widthSpin_.set_range(1, 20000);
+    widthSpin_.set_range(kMinCanvasSide, kMaxCanvasSide);
     widthSpin_.set_increments(1, 100);
     widthSpin_.set_value(1920);
-    heightSpin_.set_range(1, 20000);
+    heightSpin_.set_range(kMinCanvasSide, kMaxCanvasSide);
     heightSpin_.set_increments(1, 100);
     heightSpin_.set_value(1080);
     for (auto* spin : {&widthSpin_, &heightSpin_}) {