Backends: Vulkan: Don't skip drawing when there's no vertexes to ensure that user callbacks are still processed.

docs/CHANGELOG.txt

@@ -49,6 +49,8 @@ Other Changes:
 - Backends: Win32: Support for #define NOGDI, won't try to call GetDeviceCaps(). (#3137, #2327)
 - Backends: OpenGL: Fixed handling of GL 4.5+ glClipControl(GL_UPPER_LEFT) by inverting the
   projection matrix top and bottom values. (#3143, #3146) [@u3shit]
+- Backends: Vulkan: Fixed edge case where render callbacks wouldn't be called if the ImDrawData 
+  structure didn't have any vertices. (#2697) [@kudaba]
 
 
 -----------------------------------------------------------------------

examples/imgui_impl_vulkan.cpp

@@ -22,6 +22,7 @@
 
 // CHANGELOG
 // (minor and older changes stripped away, please see git history for details)
+//  2020-04-26: Vulkan: Fixed edge case where render callbacks wouldn't be called if the ImDrawData didn't have vertices.
 //  2019-08-01: Vulkan: Added support for specifying multisample count. Set ImGui_ImplVulkan_InitInfo::MSAASamples to one of the VkSampleCountFlagBits values to use, default is non-multisampled as before.
 //  2019-05-29: Vulkan: Added support for large mesh (64K+ vertices), enable ImGuiBackendFlags_RendererHasVtxOffset flag.
 //  2019-04-30: Vulkan: Added support for special ImDrawCallback_ResetRenderState callback to reset render state.
@@ -274,6 +275,7 @@ static void ImGui_ImplVulkan_SetupRenderState(ImDrawData* draw_data, VkCommandBu
     }
 
     // Bind Vertex And Index Buffer:
+    if (draw_data->TotalVtxCount > 0)
     {
         VkBuffer vertex_buffers[1] = { rb->VertexBuffer };
         VkDeviceSize vertex_offset[1] = { 0 };
@@ -314,7 +316,7 @@ void ImGui_ImplVulkan_RenderDrawData(ImDrawData* draw_data, VkCommandBuffer comm
     // Avoid rendering when minimized, scale coordinates for retina displays (screen coordinates != framebuffer coordinates)
     int fb_width = (int)(draw_data->DisplaySize.x * draw_data->FramebufferScale.x);
     int fb_height = (int)(draw_data->DisplaySize.y * draw_data->FramebufferScale.y);
-    if (fb_width <= 0 || fb_height <= 0 || draw_data->TotalVtxCount == 0)
+    if (fb_width <= 0 || fb_height <= 0)
         return;
 
     ImGui_ImplVulkan_InitInfo* v = &g_VulkanInitInfo;
@@ -343,6 +345,7 @@ void ImGui_ImplVulkan_RenderDrawData(ImDrawData* draw_data, VkCommandBuffer comm
         CreateOrResizeBuffer(rb->IndexBuffer, rb->IndexBufferMemory, rb->IndexBufferSize, index_size, VK_BUFFER_USAGE_INDEX_BUFFER_BIT);
 
     // Upload vertex/index data into a single contiguous GPU buffer
+    if (vertex_size > 0)
     {
         ImDrawVert* vtx_dst = NULL;
         ImDrawIdx* idx_dst = NULL;