Backends, Viewports: Vulkan: Fix the use of the incorrect fence in wait for fence. (#4208)

The fence being waited upon was not the one associated with the current frame.
This results in validation error detecting a reset of command buffers still in use and resetting fences while still in use.
Read more details in #4208

backends/imgui_impl_vulkan.cpp

@@ -1551,6 +1551,11 @@ static void ImGui_ImplVulkan_RenderWindow(ImGuiViewport* viewport, void*)
     ImGui_ImplVulkanH_Frame* fd = &wd->Frames[wd->FrameIndex];
     ImGui_ImplVulkanH_FrameSemaphores* fsd = &wd->FrameSemaphores[wd->SemaphoreIndex];
     {
+        {
+          err = vkAcquireNextImageKHR(v->Device, wd->Swapchain, UINT64_MAX, fsd->ImageAcquiredSemaphore, VK_NULL_HANDLE, &wd->FrameIndex);
+          check_vk_result(err);
+          fd = &wd->Frames[wd->FrameIndex];
+        }
         for (;;)
         {
             err = vkWaitForFences(v->Device, 1, &fd->Fence, VK_TRUE, 100);
@@ -1558,11 +1563,6 @@ static void ImGui_ImplVulkan_RenderWindow(ImGuiViewport* viewport, void*)
             if (err == VK_TIMEOUT) continue;
             check_vk_result(err);
         }
-        {
-            err = vkAcquireNextImageKHR(v->Device, wd->Swapchain, UINT64_MAX, fsd->ImageAcquiredSemaphore, VK_NULL_HANDLE, &wd->FrameIndex);
-            check_vk_result(err);
-            fd = &wd->Frames[wd->FrameIndex];
-        }
         {
             err = vkResetCommandPool(v->Device, fd->CommandPool, 0);
             check_vk_result(err);

docs/CHANGELOG.txt

@@ -126,6 +126,7 @@ Docking+Viewports Branch:
 
 - Docking: Clicking on the right-most close button of a docking node closes all windows. (#4186)
 - Docking: Fix IsWindowAppearing() and ImGuiCond_Appearing on docked windows. (#4177, #3982, #1497, #1061)
+- Backends: Vulkan: Fix the use of the incorrect fence for secondary viewports. (#4208) [@FunMiles]
 
 
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