Viewports: added ImGuiBackendFlags_HasParentViewportId backend flag. (#8948)

docs/BACKENDS.md

@@ -182,6 +182,7 @@ The Platform backends in impl_impl_XXX.cpp files contain many implementations.
   - `ImGuiBackendFlags_HasSetMousePos`: supports io.WantSetMousePos requests to reposition the OS mouse position (only used if io.ConfigNavMoveSetMousePos is set).
   - `ImGuiBackendFlags_PlatformHasViewports` supports multiple viewports. (multi-viewports only)
   - `ImGuiBackendFlags_HasMouseHoveredViewport` supports calling io.AddMouseViewportEvent() with the viewport under the mouse. IF POSSIBLE, ignore viewports with the ImGuiViewportFlags_NoInputs flag. If this cannot be done, Dear ImGui needs to use a flawed heuristic to find the viewport under mouse position, as it doesn't know about foreign windows. (multi-viewports only)
+  - `ImGuiBackendFlags_HasParentViewportId` supports honoring viewport->ParentViewportId value, by applying the corresponding parent/child relation at the Platform level.
 
 **In your `ImGui_ImplXXX_NewFrame()` function:**
 - Set `io.DeltaTime` to the time elapsed (in seconds) since last frame.

docs/CHANGELOG.txt

@@ -105,6 +105,11 @@ Docking+Viewports Branch:
 - Nav: fixed a crash that could occur when opening a popup following the processing
   of a global shortcut while no windows were focused (the fix done in 1.92.3 was
   incomplete for docking branch).
+- Viewports: added ImGuiBackendFlags_HasParentViewportId backend flag for
+  backend to specify if it can honor the viewport->ParentViewportId value by
+  applying the corresponding parent/child relation at the Platform level. (#8948)
+  - SDL3, Win32 backends: supported.
+  - SDL2, GLFW, OSX backends: unsupported.
 - Viewports: fixed a bug where ImGuiWindowFlags_NoBringToFrontOnFocus would effectivey
   be ignored when windows first appear and viewports are enabled. (#7008) [@jshofmann]
 - Viewports: changed default value of io.ConfigViewportsNoDefaultParent to true. (#8948)