Windows: loosened code to handle ImGuiButtonFlags_FlattenChildren so that master matches docking. (#8554)

This essentially merges 059560d2 back into master.

docs/CHANGELOG.txt

@@ -52,6 +52,9 @@ Other changes:
   codepath that preserve last contents size when collapsed, resulting in
   programmatically uncollapsing auto-sizing windows having them flicker size
   for a frame. (#7691) [@achabense]
+- Windows: loosened code to allow hovering of resize grips, borders, and table
+  borders while hovering a sibling child window, so that the code in master matches
+  one in docking (they accidentally diverged). (#8554)
 - Nav: fixed assertion when holding gamepad FaceLeft/West button to open
   CTRL+Tab windowing + pressing a keyboard key. (#8525)
 - Error Handling: added better error report and recovery for extraneous

imgui_widgets.cpp

@@ -518,7 +518,7 @@ bool ImGui::ButtonBehavior(const ImRect& bb, ImGuiID id, bool* out_hovered, bool
         flags |= (item_flags & ImGuiItemFlags_ButtonRepeat) ? ImGuiButtonFlags_PressedOnClick : ImGuiButtonFlags_PressedOnDefault_;
 
     ImGuiWindow* backup_hovered_window = g.HoveredWindow;
-    const bool flatten_hovered_children = (flags & ImGuiButtonFlags_FlattenChildren) && g.HoveredWindow && g.HoveredWindow->RootWindow == window;
+    const bool flatten_hovered_children = (flags & ImGuiButtonFlags_FlattenChildren) && g.HoveredWindow && g.HoveredWindow->RootWindow == window->RootWindow;
     if (flatten_hovered_children)
         g.HoveredWindow = window;