diff --git a/backends/imgui_impl_glfw.cpp b/backends/imgui_impl_glfw.cpp
index 3bafa5d1..d61d96a4 100644
--- a/backends/imgui_impl_glfw.cpp
+++ b/backends/imgui_impl_glfw.cpp
@@ -923,8 +923,11 @@ struct ImGui_ImplGlfw_ViewportData
     bool        WindowOwned;
     int         IgnoreWindowPosEventFrame;
     int         IgnoreWindowSizeEventFrame;
+#ifdef _WIN32
+    WNDPROC     PrevWndProc;
+#endif
 
-    ImGui_ImplGlfw_ViewportData()  { Window = nullptr; WindowOwned = false; IgnoreWindowSizeEventFrame = IgnoreWindowPosEventFrame = -1; }
+    ImGui_ImplGlfw_ViewportData()  { memset(this, 0, sizeof(*this)); IgnoreWindowSizeEventFrame = IgnoreWindowPosEventFrame = -1; }
     ~ImGui_ImplGlfw_ViewportData() { IM_ASSERT(Window == nullptr); }
 };
 
@@ -1059,9 +1062,8 @@ static void ImGui_ImplGlfw_ShowWindow(ImGuiViewport* viewport)
 
     // GLFW hack: install hook for WM_NCHITTEST message handler
 #if !GLFW_HAS_MOUSE_PASSTHROUGH && GLFW_HAS_WINDOW_HOVERED && defined(_WIN32)
-    ImGui_ImplGlfw_Data* bd = ImGui_ImplGlfw_GetBackendData();
     ::SetPropA(hwnd, "IMGUI_VIEWPORT", viewport);
-    IM_ASSERT(bd->PrevWndProc == (WNDPROC)::GetWindowLongPtr(hwnd, GWLP_WNDPROC));
+    vd->PrevWndProc = (WNDPROC)::GetWindowLongPtr(hwnd, GWLP_WNDPROC);
     ::SetWindowLongPtr(hwnd, GWLP_WNDPROC, (LONG_PTR)ImGui_ImplGlfw_WndProc);
 #endif
 
@@ -1263,6 +1265,12 @@ static ImGuiMouseSource GetMouseSourceFromMessageExtraInfo()
 static LRESULT CALLBACK ImGui_ImplGlfw_WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
 {
     ImGui_ImplGlfw_Data* bd = ImGui_ImplGlfw_GetBackendData();
+    WNDPROC prev_wndproc = bd->PrevWndProc;
+    ImGuiViewport* viewport = (ImGuiViewport*)::GetPropA(hWnd, "IMGUI_VIEWPORT");
+    if (viewport != NULL)
+        if (ImGui_ImplGlfw_ViewportData* vd = (ImGui_ImplGlfw_ViewportData*)viewport->PlatformUserData)
+            prev_wndproc = vd->PrevWndProc;
+
     switch (msg)
     {
         // GLFW doesn't allow to distinguish Mouse vs TouchScreen vs Pen.
@@ -1284,14 +1292,13 @@ static LRESULT CALLBACK ImGui_ImplGlfw_WndProc(HWND hWnd, UINT msg, WPARAM wPara
         // The ImGuiViewportFlags_NoInputs flag is set while dragging a viewport, as want to detect the window behind the one we are dragging.
         // If you cannot easily access those viewport flags from your windowing/event code: you may manually synchronize its state e.g. in
         // your main loop after calling UpdatePlatformWindows(). Iterate all viewports/platform windows and pass the flag to your windowing system.
-        ImGuiViewport* viewport = (ImGuiViewport*)::GetPropA(hWnd, "IMGUI_VIEWPORT");
         if (viewport && (viewport->Flags & ImGuiViewportFlags_NoInputs))
             return HTTRANSPARENT;
         break;
     }
 #endif
     }
-    return ::CallWindowProc(bd->PrevWndProc, hWnd, msg, wParam, lParam);
+    return ::CallWindowProc(prev_wndproc, hWnd, msg, wParam, lParam);
 }
 #endif // #ifdef _WIN32
 
diff --git a/docs/CHANGELOG.txt b/docs/CHANGELOG.txt
index e3b5ef52..9f6255db 100644
--- a/docs/CHANGELOG.txt
+++ b/docs/CHANGELOG.txt
@@ -126,12 +126,10 @@ Docking+Viewports Branch:
 - Docking: renamed ImGuiDockNodeFlags_NoSplit to ImGuiDockNodeFlags_NoDockingSplit.
 - Docking: renamed ImGuiDockNodeFlags_NoDockingInCentralNode to ImGuiDockNodeFlags_NoDockingOverCentralNode.
 - Docking: Internals: renamed ImGuiDockNodeFlags_NoDockingSplitMe to ImGuiDockNodeFlags_NoDockingSplit.
-
-Docking+Viewports Branch:
-
-- Docking: Fixed a bug where ClassId compare tests (when using SetNextWindowClass)
-  on success would prevent further filter from running, namely the one that prevent
-  docking over a popup.
+- Docking: Fixed a bug where ClassId compare tests (when using SetNextWindowClass) on success would
+  prevent further filter from running, namely the one that prevent docking over a popup.
+- Backends: GLFW: Fixed an assertion in situation where the WndProc handler is different between
+  main and secondary viewport (may happen due to third-party hooks). (#6889)
 
 
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