Easier identification of ImGui windows on X11

[Ikos3k]

Version/Branch of Dear ImGui:

Version 1.91.7 WIP, Branch: docking

Back-ends:

imgui_impl_glfw.cpp + imgui_impl_opengl3.cpp

Compiler, OS:

Linux i3wm (x11)

Full config/build information:

Dear ImGui 1.91.7 WIP (19163)
--------------------------------
sizeof(size_t): 8, sizeof(ImDrawIdx): 2, sizeof(ImDrawVert): 20
define: __cplusplus=201703
define: __linux__
define: __GNUC__=14
define: IMGUI_HAS_VIEWPORT
define: IMGUI_HAS_DOCK
--------------------------------
io.BackendPlatformName: imgui_impl_glfw
io.BackendRendererName: imgui_impl_opengl3
io.ConfigFlags: 0x00000480
 DockingEnable
 ViewportsEnable
io.ConfigViewportsNoDecoration
io.ConfigNavCaptureKeyboard
io.ConfigInputTextCursorBlink
io.ConfigWindowsResizeFromEdges
io.ConfigMemoryCompactTimer = 60.0
io.BackendFlags: 0x00001C0E
 HasMouseCursors
 HasSetMousePos
 PlatformHasViewports
 HasMouseHoveredViewport
 RendererHasVtxOffset
 RendererHasViewports
--------------------------------
io.Fonts: 1 fonts, Flags: 0x00000000, TexSize: 512,128
io.DisplaySize: 800.00,600.00
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:

My Issue:
Managing ImGui windows under i3wm (or other tiling window managers) is problematic because all ImGui windows either lack sufficient unique identifiers or share a generic WM_CLASS property. This limitation makes it difficult to apply specific window management rules, such as automatically enabling floating mode for docked ImGui windows. For example, in i3wm’s configuration file, users would like to apply a rule like:
for_window [class="ImGui"] floating enable
for_window [instance="ImGui"] floating enable

Proposed Solution:
ImGui could set a custom WM_CLASS property or another identifier for their windows on X11, allowing for easier and more precise window management in tiling window managers like i3wm.

Screenshots/Video:

2025-01-03.23-20-38.mp4

xprop output of ImGui docked window:

Minimal, Complete and Verifiable Example code:

ImGui::Begin("Example Bug");
ImGui::End();

[ocornut]

Hello,
I don't know how to implement this but it would be great it would largely help with #2117
A PR would be welcome.