MovingWindow: Track click offset based on MovingWindow and not its RootWindow. Followup to 3849bb4470. Comments + adding a local to ease patch #1345.

imgui.cpp

@@ -2392,10 +2392,17 @@ void ImGui::NewFrame()
         IM_ASSERT(g.MovingWindow->MoveId == g.MovingWindowMoveId);
         if (g.IO.MouseDown[0])
         {
+            // MovingWindow = window we clicked on, could be a child window. We track it to preserve Focus and so that ActiveIdWindow == MovingWindow and ActiveId == MovingWindow->MoveId for consistency.
+            // actually_moving_window = MovingWindow->RootWindow.
+            ImGuiWindow* actually_moving_window = g.MovingWindow->RootWindow;
             ImVec2 pos = g.IO.MousePos - g.ActiveIdClickOffset;
-            if (g.MovingWindow->RootWindow->PosFloat.x != pos.x || g.MovingWindow->RootWindow->PosFloat.y != pos.y)
-                MarkIniSettingsDirty(g.MovingWindow->RootWindow);
-            g.MovingWindow->RootWindow->PosFloat = pos;
+            if (actually_moving_window != g.MovingWindow)
+                pos += actually_moving_window->PosFloat - g.MovingWindow->PosFloat;
+            if (actually_moving_window->PosFloat.x != pos.x || actually_moving_window->PosFloat.y != pos.y)
+            {
+                MarkIniSettingsDirty(actually_moving_window);
+                actually_moving_window->PosFloat = pos;
+            }
             FocusWindow(g.MovingWindow);
         }
         else
@@ -2950,7 +2957,7 @@ void ImGui::EndFrame()
                     // Set ActiveId even if the _NoMove flag is set, without it dragging away from a window with _NoMove would activate hover on other windows.
                     FocusWindow(g.HoveredWindow);
                     SetActiveID(g.HoveredWindow->MoveId, g.HoveredWindow);
-                    g.ActiveIdClickOffset = g.IO.MousePos - g.HoveredRootWindow->Pos;
+                    g.ActiveIdClickOffset = g.IO.MousePos - g.HoveredWindow->Pos;
                     if (!(g.HoveredWindow->Flags & ImGuiWindowFlags_NoMove) && !(g.HoveredRootWindow->Flags & ImGuiWindowFlags_NoMove))
                     {
                         g.MovingWindow = g.HoveredWindow;

imgui_internal.h

@@ -534,7 +534,7 @@ struct ImGuiContext
     bool                    ActiveIdAllowOverlap;               // Active widget allows another widget to steal active id (generally for overlapping widgets, but not always)
     ImVec2                  ActiveIdClickOffset;                // Clicked offset from upper-left corner, if applicable (currently only set by ButtonBehavior)
     ImGuiWindow*            ActiveIdWindow;
-    ImGuiWindow*            MovingWindow;                       // Track the child window we clicked on to move a window.
+    ImGuiWindow*            MovingWindow;                       // Track the window we clicked on (in order to preserve focus). The actually window that is moved is generally MovingWindow->RootWindow.
     ImGuiID                 MovingWindowMoveId;                 // == MovingWindow->MoveId
     ImVector<ImGuiColMod>   ColorModifiers;                     // Stack for PushStyleColor()/PopStyleColor()
     ImVector<ImGuiStyleMod> StyleModifiers;                     // Stack for PushStyleVar()/PopStyleVar()