What Are ImGuiWindowFlags and How to Use Them in Dear ImGui

ImGuiWindowFlags are bit-field constants defined in imgui.h that control the appearance, sizing, and interaction model of ImGui windows, which you combine with the bitwise OR operator (|) and pass to ImGui::Begin() or ImGui::BeginChild().

In the ocornut/imgui repository, ImGuiWindowFlags provide the primary mechanism for customizing how a window behaves when created with ImGui::Begin(). These flags let you remove title bars, disable resizing, enable auto-fit, and fine-tune navigation without subclassing or overriding rendering code.

Where ImGuiWindowFlags Are Defined

In imgui.h (around line 1171), Dear ImGui declares the ImGuiWindowFlags_ enum and exposes it through the ImGuiWindowFlags type. Each constant is defined as a single bit shift (1 << n), so multiple flags can be merged into one bitmask without overlap.

Common ImGuiWindowFlags Values

The public API exposes a broad set of window-level toggles. The most frequently used ImGuiWindowFlags include:

• ImGuiWindowFlags_NoTitleBar — Hides the title bar; useful for borderless pop-up panels.
• ImGuiWindowFlags_NoResize — Removes resize grips and prevents manual resizing.
• ImGuiWindowFlags_NoMove — Locks the window in place, which is ideal for HUD elements.
• ImGuiWindowFlags_NoScrollbar — Hides scrollbars while still allowing programmatic scrolling.
• ImGuiWindowFlags_NoCollapse — Removes the collapse button from the title bar.
• ImGuiWindowFlags_AlwaysAutoResize — Forces the window to fit its contents every single frame.
• ImGuiWindowFlags_MenuBar — Allocates extra space for a menu bar via ImGui::BeginMenuBar().
• ImGuiWindowFlags_NoBackground — Skips background drawing, producing a transparent overlay effect.
• ImGuiWindowFlags_NoFocusOnAppearing — Prevents the window from stealing focus when it is first shown.
• ImGuiWindowFlags_NoBringToFrontOnFocus — Keeps the window at its current Z-order even when focused.
• ImGuiWindowFlags_HorizontalScrollbar — Adds a horizontal scrollbar for wide content.
• ImGuiWindowFlags_NoNav — Disables keyboard and gamepad navigation entirely inside the window.

How to Combine and Apply ImGuiWindowFlags

You pass the combined bitmask as the third argument of ImGui::Begin() or via the optional window_flags parameter of ImGui::BeginChild(). To request default behavior, pass 0 or ImGuiWindowFlags_None.

Fixed-Size Tool Window Example

ImGui::Begin("Fixed", nullptr,
             ImGuiWindowFlags_NoTitleBar |
             ImGuiWindowFlags_NoResize |
             ImGuiWindowFlags_NoScrollbar);
ImGui::Text("This window cannot be moved, resized or scrolled.");
ImGui::End();

Auto-Resizing Debug Panel Example

ImGui::Begin("Auto-Resize", nullptr,
             ImGuiWindowFlags_AlwaysAutoResize |
             ImGuiWindowFlags_NoCollapse);
for (int i = 0; i < 10; ++i)
    ImGui::Text("Line %d", i);
ImGui::End();

Child Window with Flags Example

ImGui::Begin("Parent");
ImGui::BeginChild("Child", ImVec2(0, ImGui::GetContentRegionAvail().y),
                   ImGuiChildFlags_Borders |
                   ImGuiChildFlags_AutoResizeX);
ImGui::Text("Child window expands horizontally.");
ImGui::EndChild();
ImGui::End();

Internal Flags to Avoid

Dear ImGui reserves several flags for its own window management logic in imgui.cpp. These include ImGuiWindowFlags_ChildWindow, ImGuiWindowFlags_Tooltip, and ImGuiWindowFlags_Popup. Because they drive internal behavior and are not part of the public API, application code should never set them manually; doing so can create silent bugs or break compatibility with future releases.

Key Source Files

• imgui.h — Declares the ImGuiWindowFlags_ enum and the ImGui::Begin() / ImGui::BeginChild() signatures that accept the flags.
• imgui.cpp — Contains the core implementation that interprets the bitmask and applies the corresponding drawing and interaction rules.
• imgui_demo.cpp — Provides runnable demonstrations; search for Begin( to see how different flag combinations behave in practice.

Summary

• ImGuiWindowFlags are bit-field constants stored in the ImGuiWindowFlags type, declared in imgui.h.
• Combine multiple flags with the bitwise OR operator (|) and pass the result to ImGui::Begin() or ImGui::BeginChild().
• Use 0 or ImGuiWindowFlags_None when you want the default window chrome and behavior.
• Avoid internal flags such as ImGuiWindowFlags_ChildWindow because they are reserved for internal use inside imgui.cpp.

Frequently Asked Questions

How do I combine multiple ImGuiWindowFlags?

Combine individual constants with the bitwise OR operator (|). Because each value occupies a unique bit, OR-ing them produces a safe bitmask that ImGui::Begin() interprets correctly.

Can I use ImGuiWindowFlags with ImGui::BeginChild()?

Yes. ImGui::BeginChild() accepts an optional ImGuiWindowFlags parameter, so child windows can be configured with the same window-level flags as top-level windows.

What happens if I pass 0 or ImGuiWindowFlags_None?

Passing zero or the explicit ImGuiWindowFlags_None constant yields the default window behavior, including a title bar, resize grips, scrollbars, and collapse button.

Are internal flags such as ImGuiWindowFlags_ChildWindow safe to use?

No. Flags like ImGuiWindowFlags_ChildWindow, ImGuiWindowFlags_Tooltip, and ImGuiWindowFlags_Popup are reserved for Dear ImGui's internal implementation. Using them in your application code is unsupported and may break in future versions.

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