Merged a bunch of small changes from Docking branch to reduce the difference between branches.

Noticeable: horizontal alignment of CloseButton. Menu fill take account of border. Various stylistic tweaks to accomodate other changes in Docking.

docs/CHANGELOG.txt

@@ -50,6 +50,7 @@ Other Changes:
 - GetMouseDragDelta(): verify that mouse positions are valid otherwise returns zero.
 - Inputs: Also add support for horizontal scroll with Shift+Mouse Wheel. (#2424, #1463) [@LucaRood]
 - PlotLines, PlotHistogram: Ignore NaN values when calculating min/max bounds. (#2485)
+- Window: Window close button is horizontally aligned with style.FramePadding.x.
 - Misc: Added IM_MALLOC/IM_FREE macros mimicking IM_NEW/IM_DELETE so user doesn't need to revert
   to using the ImGui::MemAlloc()/MemFree() calls directly.
 - Examples: OpenGL: Added a dummy GL call + comments in ImGui_ImplOpenGL3_Init() to detect uninitialized

imgui_demo.cpp

@@ -4267,16 +4267,16 @@ void ShowExampleAppDocuments(bool* p_open)
 {
     static ExampleAppDocuments app;
 
+    // Options
+    static bool opt_reorderable = true;
+    static ImGuiTabBarFlags opt_fitting_flags = ImGuiTabBarFlags_FittingPolicyDefault_;
+
     if (!ImGui::Begin("Example: Documents", p_open, ImGuiWindowFlags_MenuBar))
     {
         ImGui::End();
         return;
     }
 
-    // Options
-    static bool opt_reorderable = true;
-    static ImGuiTabBarFlags opt_fitting_flags = ImGuiTabBarFlags_FittingPolicyDefault_;
-
     // Menu
     if (ImGui::BeginMenuBar())
     {

imgui_internal.h

@@ -1221,7 +1221,7 @@ struct IMGUI_API ImGuiWindow
     bool                    WantCollapseToggle;
     bool                    SkipItems;                          // Set when items can safely be all clipped (e.g. window not visible or collapsed)
     bool                    Appearing;                          // Set during the frame where the window is appearing (or re-appearing)
-    bool                    Hidden;                             // Do not display (== (HiddenFramesForResize > 0) ||
+    bool                    Hidden;                             // Do not display (== (HiddenFrames*** > 0))
     bool                    HasCloseButton;                     // Set when the window has a close button (p_open != NULL)
     signed char             ResizeBorderHeld;                   // Current border being held for resize (-1: none, otherwise 0-3)
     short                   BeginCount;                         // Number of Begin() during the current frame (generally 0 or 1, 1+ if appending via multiple Begin/End pairs)