Backends: GLFW: Handle spurious leave/enter sequences apparently subm…

…itted by X11/GLFW combo. (#4984)

backends/imgui_impl_glfw.cpp

@@ -91,6 +91,7 @@ struct ImGui_ImplGlfw_Data
     double                  Time;
     GLFWwindow*             MouseWindow;
     GLFWcursor*             MouseCursors[ImGuiMouseCursor_COUNT];
+    ImVec2                  LastMousePos;
     bool                    InstalledCallbacks;
 
     // Chain GLFW callbacks: our callbacks will call the user's previously installed callbacks, if any.
@@ -338,8 +339,11 @@ void ImGui_ImplGlfw_CursorPosCallback(GLFWwindow* window, double x, double y)
 
     ImGuiIO& io = ImGui::GetIO();
     io.AddMousePosEvent((float)x, (float)y);
+    bd->LastMousePos = ImVec2((float)x, (float)y);
 }
 
+// Workaround: X11 seems to send spurious Leave/Enter events which would make us lose our position,
+// so we back it up and restore on Leave/Enter (see https://github.com/ocornut/imgui/issues/4984)
 void ImGui_ImplGlfw_CursorEnterCallback(GLFWwindow* window, int entered)
 {
     ImGui_ImplGlfw_Data* bd = ImGui_ImplGlfw_GetBackendData();
@@ -348,9 +352,13 @@ void ImGui_ImplGlfw_CursorEnterCallback(GLFWwindow* window, int entered)
 
     ImGuiIO& io = ImGui::GetIO();
     if (entered)
+    {
         bd->MouseWindow = window;
-    if (!entered && bd->MouseWindow == window)
+        io.AddMousePosEvent(bd->LastMousePos.x, bd->LastMousePos.y);
+    }
+    else if (!entered && bd->MouseWindow == window)
     {
+        bd->LastMousePos = io.MousePos;
         bd->MouseWindow = NULL;
         io.AddMousePosEvent(-FLT_MAX, -FLT_MAX);
     }