SDL3 backend: phantom window frame on viewport creation when NoDecoration is disabled

[RT2Code]

Version/Branch of Dear ImGui:

Version 1.91.7 Branch: docking

Back-ends:

imgui_impl_sdl3.cpp + imgui_impl_opengl3.cpp

Compiler, OS:

Windows 11 + MSVC 22

Full config/build information:

Dear ImGui 1.91.7 (19170)
--------------------------------
sizeof(size_t): 8, sizeof(ImDrawIdx): 2, sizeof(ImDrawVert): 20
define: __cplusplus=199711
define: _WIN32
define: _WIN64
define: _MSC_VER=1942
define: _MSVC_LANG=201402
define: IMGUI_HAS_VIEWPORT
define: IMGUI_HAS_DOCK
--------------------------------
io.BackendPlatformName: imgui_impl_sdl3
io.BackendRendererName: imgui_impl_dx11
io.ConfigFlags: 0x00000483
 NavEnableKeyboard
 NavEnableGamepad
 DockingEnable
 ViewportsEnable
io.ConfigViewportsNoAutoMerge
io.ConfigViewportsNoTaskBarIcon
io.ConfigViewportsNoDecoration
io.ConfigInputTextCursorBlink
io.ConfigWindowsResizeFromEdges
io.ConfigMemoryCompactTimer = 60.0
io.BackendFlags: 0x00001C0E
 HasMouseCursors
 HasSetMousePos
 PlatformHasViewports
 HasMouseHoveredViewport
 RendererHasVtxOffset
 RendererHasViewports
--------------------------------
io.Fonts: 4 fonts, Flags: 0x00000000, TexSize: 512,256
io.DisplaySize: 800.00,1000.00
io.DisplayFramebufferScale: 1.00,1.00
--------------------------------
style.WindowPadding: 8.00,8.00
style.WindowBorderSize: 1.00
style.FramePadding: 4.00,3.00
style.FrameRounding: 0.00
style.FrameBorderSize: 0.00
style.ItemSpacing: 8.00,4.00
style.ItemInnerSpacing: 4.00,4.00

Details:

When the NoDecoration flag is disabled and a window is dragged outside the main viewport, the frame of the newly created window appears for one frame in the top-left corner of the screen.

Screenshots/Video:

Screen.Recording.2024-09-17.195208-1.mp4

Minimal, Complete and Verifiable Example code:

io.ConfigViewportsNoAutoMerge = false;
io.ConfigViewportsNoDecoration= false;

[PathogenDavid]

I didn't have a change to test, but I suspect this happens because the SDL3 backend is missing SDL_WINDOW_HIDDEN here:

imgui/backends/imgui_impl_sdl3.cpp

Lines 958 to 965 in 109dd2b

	SDL_WindowFlags sdl_flags = 0;
	sdl_flags |= use_opengl ? SDL_WINDOW_OPENGL : (bd->UseVulkan ? SDL_WINDOW_VULKAN : 0);
	sdl_flags |= SDL_GetWindowFlags(bd->Window) & SDL_WINDOW_HIGH_PIXEL_DENSITY;
	sdl_flags |= (viewport->Flags & ImGuiViewportFlags_NoDecoration) ? SDL_WINDOW_BORDERLESS : 0;
	sdl_flags |= (viewport->Flags & ImGuiViewportFlags_NoDecoration) ? 0 : SDL_WINDOW_RESIZABLE;
	sdl_flags |= (viewport->Flags & ImGuiViewportFlags_NoTaskBarIcon) ? SDL_WINDOW_UTILITY : 0;
	sdl_flags |= (viewport->Flags & ImGuiViewportFlags_TopMost) ? SDL_WINDOW_ALWAYS_ON_TOP : 0;
	vd->Window = SDL_CreateWindow("No Title Yet", (int)viewport->Size.x, (int)viewport->Size.y, sdl_flags);

Which is used in the SDL2 backend, probably for this reason:

imgui/backends/imgui_impl_sdl2.cpp

Line 992 in 109dd2b

sdl_flags |= SDL_WINDOW_HIDDEN;

[ocornut]

This is really bizarre indeed, it has been missing from a526ff8...I wonder if at the time it was missing of SDL3's wip CreateWindow didn't show the window. Rémy could you confirm if that's your issue? (as I cannot repro here). Thanks David!

[RT2Code]

This issue is not visually consistent, I can see it most of the time I drag a viewport outside the main window, but not always. I tried, and it still happens with SDL_WINDOW_HIDDEN, albeit less often.

[ocornut]

I'll add the SDL_WINDOW_HIDDEN flag either way for now.