altsnap_jitter_test/main.cpp

// altsnap_jitter_test — automated regression test for the multi-viewport
// jitter that AltSnap (and other Win32 window movers) trigger on the C++
// app shell. The test spawns the standard fn::run_app with viewports ON,
// then drives the main GLFW window with glfwSetWindowPos every frame to
// emulate an external mover (AltSnap, tiling WM, snap-assist, ...). For
// each frame it samples the actual OS window position and ImGui's main
// viewport->Pos, and asserts both stay in sync within a 1-pixel tolerance
// after a 1-frame settle window. Without the anti-jitter patch in
// app_base.cpp the two diverge and the test exits 1.
//
// Linux/WSL: runs the same loop. With xvfb the window manager honors
// glfwSetWindowPos, so the test is meaningful in CI. On a real WSL X
// display some compositors clamp positions; the test logs and accepts
// "OS clamped" frames as long as ImGui tracks the clamped value.
//
// Windows (target): cross-compiled via build_cpp_windows, deployed to
// the Desktop apps dir, launched via cmd.exe; this is where the original
// AltSnap symptom lives, so any divergence here is the real regression.

#include "app_base.h"
#include "imgui.h"
#include <GLFW/glfw3.h>
#define GLFW_EXPOSE_NATIVE_WIN32
#ifdef _WIN32
  #include <GLFW/glfw3native.h>
#endif

#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <algorithm>

namespace {

struct Sample {
    int   frame      = 0;
    int   target_x   = 0;
    int   target_y   = 0;
    int   actual_x   = 0;
    int   actual_y   = 0;
    float vp_x       = 0.0f;
    float vp_y       = 0.0f;
};

constexpr int   kWarmupFrames     = 8;     // wait for the platform to settle
constexpr int   kTestFrames       = 60;    // measured frames
constexpr int   kStepPx           = 4;     // px / frame
constexpr int   kBaseX            = 200;
constexpr int   kBaseY            = 200;
constexpr float kSyncTolerance    = 1.0f;  // ImGui viewport vs OS pos
constexpr int   kClampTolerance   = 6;     // OS-applied vs target (compositor clamp ok up to this)

GLFWwindow* g_window     = nullptr;
int         g_frame      = 0;
int         g_max_sync   = 0;
int         g_max_clamp  = 0;
int         g_bad_sync   = 0;   // frames where vp diverges from OS pos > tolerance
int         g_bad_clamp  = 0;   // frames where OS pos diverges from target > clamp tolerance
bool        g_done       = false;

void render() {
    // Locate the main window once. ImGui_ImplGlfw stashes it on the main
    // viewport's PlatformHandle right after init.
    if (!g_window) {
        if (ImGuiViewport* vp = ImGui::GetMainViewport()) {
            g_window = (GLFWwindow*)vp->PlatformHandle;
        }
    }
    if (!g_window) return;

    if (g_frame < kWarmupFrames) {
        ++g_frame;
        return;
    }

    int rel = g_frame - kWarmupFrames;
    if (rel < kTestFrames) {
        // Drive the OS window from inside the render loop. Mimics what an
        // external mover (AltSnap, tiling WM) would do: a fresh SetWindowPos
        // every tick. With the fix in place ImGui's main viewport->Pos must
        // follow within the same frame because of the GLFW pos callback.
        int tx = kBaseX + rel * kStepPx;
        int ty = kBaseY;
        glfwSetWindowPos(g_window, tx, ty);

        // Sample AFTER the SetWindowPos. The callback fires synchronously on
        // most platforms; if not, the per-frame sync pass in app_base.cpp
        // covers it before this render() call on the next frame. We measure
        // both here for diagnostics.
        int ax = 0, ay = 0;
        glfwGetWindowPos(g_window, &ax, &ay);
        ImVec2 vp = ImGui::GetMainViewport()->Pos;

        int sync_dx  = (int)std::abs(vp.x - (float)ax);
        int sync_dy  = (int)std::abs(vp.y - (float)ay);
        int clamp_dx = std::abs(ax - tx);
        int clamp_dy = std::abs(ay - ty);
        int sync_d   = std::max(sync_dx, sync_dy);
        int clamp_d  = std::max(clamp_dx, clamp_dy);

        if ((float)sync_d > kSyncTolerance) ++g_bad_sync;
        if (clamp_d > kClampTolerance)      ++g_bad_clamp;
        g_max_sync  = std::max(g_max_sync,  sync_d);
        g_max_clamp = std::max(g_max_clamp, clamp_d);

        // Verbose first/last frame for log readability.
        if (rel == 0 || rel == kTestFrames - 1 || (rel % 10 == 0)) {
            std::fprintf(stdout,
                "[altsnap_jitter] f=%d target=(%d,%d) actual=(%d,%d) vp=(%.1f,%.1f) sync_d=%d clamp_d=%d\n",
                rel, tx, ty, ax, ay, vp.x, vp.y, sync_d, clamp_d);
            std::fflush(stdout);
        }
        ++g_frame;
        return;
    }

    if (!g_done) {
        std::fprintf(stdout,
            "[altsnap_jitter] DONE frames=%d max_sync_divergence=%dpx max_clamp_divergence=%dpx bad_sync=%d bad_clamp=%d\n",
            kTestFrames, g_max_sync, g_max_clamp, g_bad_sync, g_bad_clamp);
        std::fflush(stdout);
        g_done = true;
    }
    glfwSetWindowShouldClose(g_window, GLFW_TRUE);
}

} // namespace

int main(int argc, char** argv) {
    (void)argc; (void)argv;

    fn::AppConfig cfg;
    cfg.title          = "altsnap_jitter_test";
    cfg.width          = 800;
    cfg.height         = 600;
    cfg.vsync          = false;       // run as fast as possible
    cfg.viewports      = true;        // exact config that exhibits the bug
    cfg.init_gl_loader = false;

    int rc = fn::run_app(cfg, render);
    if (rc != 0) {
        std::fprintf(stderr, "[altsnap_jitter] run_app returned %d\n", rc);
        return rc;
    }

    // Pass criterion: every measured frame must have ImGui viewport tracking
    // the OS-reported window pos within 1px. A frame where the OS clamped
    // the position (compositor) is fine as long as ImGui follows the clamp.
    bool pass = (g_bad_sync == 0);
    std::fprintf(stdout, "[altsnap_jitter] %s\n", pass ? "PASS" : "FAIL");
    return pass ? 0 : 1;
}