When ImGuiChildFlags_AlwaysUseWindowPadding is enabled, the left and right padding widths of the Child are different, but the Parent window is the same

[o-3-o]

Version/Branch of Dear ImGui:

Version 1.90.6 WIP, Branch: docking (master/docking/etc.)

Back-ends:

imgui_impl_Win32.cpp + imgui_impl_DX11.cpp

Compiler, OS:

Win11 + MSVC 2022

Full config/build information:

Dear ImGui 1.90.6 WIP (19052)
--------------------------------
sizeof(size_t): 8, sizeof(ImDrawIdx): 2, sizeof(ImDrawVert): 20
define: __cplusplus=199711
define: IMGUI_DISABLE_OBSOLETE_FUNCTIONS
define: IMGUI_DISABLE_OBSOLETE_KEYIO
define: _WIN32
define: _WIN64
define: _MSC_VER=1939
define: _MSVC_LANG=201402
define: IMGUI_HAS_VIEWPORT
define: IMGUI_HAS_DOCK
--------------------------------
io.BackendPlatformName: imgui_impl_win32
io.BackendRendererName: imgui_impl_dx11
io.ConfigFlags: 0x0000C443
 NavEnableKeyboard
 NavEnableGamepad
 DockingEnable
 ViewportsEnable
 DpiEnableScaleViewports
 DpiEnableScaleFonts
io.ConfigViewportsNoAutoMerge
io.ConfigViewportsNoDecoration
io.ConfigViewportsNoDefaultParent
io.ConfigDockingTransparentPayload
io.ConfigInputTextCursorBlink
io.ConfigWindowsResizeFromEdges
io.ConfigMemoryCompactTimer = 60.0
io.BackendFlags: 0x00001C0E
 HasMouseCursors
 HasSetMousePos
 PlatformHasViewports
 HasMouseHoveredViewport
 RendererHasVtxOffset
 RendererHasViewports
--------------------------------
io.Fonts: 1 fonts, Flags: 0x00000000, TexSize: 512,64
io.DisplaySize: 3440.00,1392.00
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/Question:

When [ImGuiChildFlags_AlwaysUseWindowPadding] is enabled, the left and right padding widths of the Child are different, but the Parent window is the same

Screenshots/Video:

Minimal, Complete and Verifiable Example code:

ImGui::PushStyleVar(ImGuiStyleVar_WindowPadding, ImVec2(20.0f, 20.0f));
ImGui::Begin("fafafaefaefe");
	ImGui::PushStyleColor(ImGuiCol_ChildBg, IM_COL32(204, 205, 209, 255));
	ImGui::BeginChild("##Container", ImVec2(0.0f, 0.0f), ImGuiChildFlags_AlwaysUseWindowPadding);
		ImGui::Button("fafafqwfwqfqwfwqfe");
	ImGui::EndChild();
	ImGui::PopStyleColor();
ImGui::End();
ImGui::PopStyleVar();

[cfillion]

The inner clipping rectangle of all windows (child or not) is always larger than the padding (by half).

The actual padding is the same on the left and right sides, which you can observe by setting ImGuiChildFlags_AutoResizeX. Or "Show window rectangles" in ShowMetricsWindow() under "Tools".

ImVec2(0.0f, 0.0f) makes the child window use all available width in the work rectangle which includes the full padding.

imgui/imgui.cpp

Lines 6824 to 6836 in a60387a

	// Inner clipping rectangle.
	// Will extend a little bit outside the normal work region.
	// This is to allow e.g. Selectable or CollapsingHeader or some separators to cover that space.
	// Force round operator last to ensure that e.g. (int)(max.x-min.x) in user's render code produce correct result.
	// Note that if our window is collapsed we will end up with an inverted (~null) clipping rectangle which is the correct behavior.
	// Affected by window/frame border size. Used by:
	// - Begin() initial clip rect
	float top_border_size = (((flags & ImGuiWindowFlags_MenuBar) || !(flags & ImGuiWindowFlags_NoTitleBar)) ? style.FrameBorderSize : window->WindowBorderSize);
	window->InnerClipRect.Min.x = ImTrunc(0.5f + window->InnerRect.Min.x + ImMax(ImTrunc(window->WindowPadding.x * 0.5f), window->WindowBorderSize));
	window->InnerClipRect.Min.y = ImTrunc(0.5f + window->InnerRect.Min.y + top_border_size);
	window->InnerClipRect.Max.x = ImTrunc(0.5f + window->InnerRect.Max.x - ImMax(ImTrunc(window->WindowPadding.x * 0.5f), window->WindowBorderSize));
	window->InnerClipRect.Max.y = ImTrunc(0.5f + window->InnerRect.Max.y - window->WindowBorderSize);
	window->InnerClipRect.ClipWithFull(host_rect);

[o-3-o]

Thanks for @cfillion answer, now I realize that it is always larger than the padding (by half). It was a visual misunderstanding.

[o-3-o]

can solve this visual problem by using PushClipRect to limit it yourself.

ImGui::PushClipRect(ImGui::GetCurrentWindow()->Pos + ImGui::GetStyle().WindowPadding, ImGui::GetCurrentWindow()->Pos - ImGui::GetStyle().WindowPadding + ImGui::GetCurrentWindow()->Size, false);
//...
ImGui::PopClipRect();

[ocornut]

Linking to #3312 as I've been considering changing this and make the clip rect fill entire inner rect. I'll post here too if this changes.

[ocornut]

I went and pushed the change discussed in #3312 I think it's for the best 0b30947
Also generally makes it easier to draw stuff in edge of windows without requiring a clip rect change.