Windows: BeginChild(): fixed a glitch when during a resize of a child…

… window which is tightly close to the boundaries of its parent. (#7706)

docs/CHANGELOG.txt

@@ -69,6 +69,9 @@ Other changes:
 - IO: Added io.ClearInputMouse() to clear mouse state. (#4921)
 - Inputs: fixed using Shortcut() or SetNextItemShortcut() within a disabled block bypassing
   the disabled state. (#7726)
+- Windows: BeginChild(): fixed a glitch when during a resize of a child window which is
+  tightly close to the boundaries of its parent (e.g. with zero WindowPadding), the child
+  position could have temporarily be moved around by erroneous padding application. (#7706)
 - TabBar, Style: added ImGuiTabBarFlags_DrawSelectedOverline option to draw an horizontal
   line over selected tabs to increase visibility. This is used by docking.
   Added corresponding ImGuiCol_TabSelectedOverline and ImGuiCol_TabDimmedSelectedOverline colors.

imgui.cpp

@@ -6140,12 +6140,13 @@ static int ImGui::UpdateWindowManualResize(ImGuiWindow* window, const ImVec2& si
             border_target = ImClamp(border_target, clamp_min, clamp_max);
             if (flags & ImGuiWindowFlags_ChildWindow) // Clamp resizing of childs within parent
             {
-                ImGuiWindowFlags parent_flags = window->ParentWindow->Flags;
-                ImRect border_limit_rect = window->ParentWindow->InnerRect;
-                border_limit_rect.Expand(ImVec2(-ImMax(window->WindowPadding.x, window->WindowBorderSize), -ImMax(window->WindowPadding.y, window->WindowBorderSize)));
-                if ((parent_flags & (ImGuiWindowFlags_HorizontalScrollbar | ImGuiWindowFlags_AlwaysHorizontalScrollbar)) == 0 || (parent_flags & ImGuiWindowFlags_NoScrollbar))
+                ImGuiWindow* parent_window = window->ParentWindow;
+                ImGuiWindowFlags parent_flags = parent_window->Flags;
+                ImRect border_limit_rect = parent_window->InnerRect;
+                border_limit_rect.Expand(ImVec2(-ImMax(parent_window->WindowPadding.x, parent_window->WindowBorderSize), -ImMax(parent_window->WindowPadding.y, parent_window->WindowBorderSize)));
+                if ((axis == ImGuiAxis_X) && ((parent_flags & (ImGuiWindowFlags_HorizontalScrollbar | ImGuiWindowFlags_AlwaysHorizontalScrollbar)) == 0 || (parent_flags & ImGuiWindowFlags_NoScrollbar)))
                     border_target.x = ImClamp(border_target.x, border_limit_rect.Min.x, border_limit_rect.Max.x);
-                if (parent_flags & ImGuiWindowFlags_NoScrollbar)
+                if ((axis == ImGuiAxis_Y) && (parent_flags & ImGuiWindowFlags_NoScrollbar))
                     border_target.y = ImClamp(border_target.y, border_limit_rect.Min.y, border_limit_rect.Max.y);
             }
             if (!ignore_resize)