Viewport windows: rendering issue when using transparent title bar color under macOS / Metal or Vulkan

[pthom]

Version/Branch of Dear ImGui:

Version: v1.90.0
Branch: docking

Back-end/Renderer/Compiler/OS

Back-ends: Metal or Vulkan + SDL or Glfw
Operating System: macOS

My Issue/Question:

Viewport windows' title may show rendering issues when the title bar color is partially transparent, when using Metal or Vulkan.

This happens on the docking branch, with viewports enabled, when a window is dragged out of the main window.

Stability is not affected, and a simple workaround is to not use transparent colors for the title bar.

Screenshots/Video

(link to giphy source if the above screenshot does not work)

Standalone, minimal, complete and verifiable example:

Add this:

    style.Colors[ImGuiCol_TitleBg].w = 0.5f;
    style.Colors[ImGuiCol_TitleBgActive].w = 0.5f;
    style.Colors[ImGuiCol_TitleBgCollapsed].w = 0.5f;

before the main loop inside examples/example_glfw_vulkan/main.cpp or examples/example_glfw_metal/main.mm

[PathogenDavid]

This is half unimplemented feature half bug. The glitchiness you're seeing is the gabled stale contents of the back buffer.

The unimplemented feature is that transparency is not supported for secondary viewports. (It's a non-trivial problem, see #2766)

The bug is that the alpha isn't being ignored like it is for the window background or collapsed window title bar. I've submitted a fix here: #7184

[ocornut]

Fixed indeed, thank you David.

We set ImGuiViewportFlags_NoRendererClear to instruct backend that they can avoid clearing the framebuffer, and full-transparency support will not be able to set this flag but it's a nice little optimization for when we can.

[pthom]

Many thanks for the quick fix!