Selectable: Fixed highlight/hit extent when used with horizontal scrolling (in or outside columns). (#3187, #3386)

# Conflicts:
#	imgui_widgets.cpp

imgui_internal.h

@@ -1015,7 +1015,7 @@ struct ImGuiColumns
     float               HostCursorMaxPosX;      // Backup of CursorMaxPos at the time of BeginColumns()
     ImRect              HostInitialClipRect;    // Backup of ClipRect at the time of BeginColumns()
     ImRect              HostBackupClipRect;     // Backup of ClipRect during PushColumnsBackground()/PopColumnsBackground()
-    ImRect              HostWorkRect;           // Backup of WorkRect at the time of BeginColumns()
+    ImRect              HostBackupParentWorkRect;//Backup of WorkRect at the time of BeginColumns()
     ImVector<ImGuiColumnData> Columns;
     ImDrawListSplitter  Splitter;
 
@@ -1612,7 +1612,8 @@ struct IMGUI_API ImGuiWindow
     ImRect                  OuterRectClipped;                   // == Window->Rect() just after setup in Begin(). == window->Rect() for root window.
     ImRect                  InnerRect;                          // Inner rectangle (omit title bar, menu bar, scroll bar)
     ImRect                  InnerClipRect;                      // == InnerRect shrunk by WindowPadding*0.5f on each side, clipped within viewport or parent clip rect.
-    ImRect                  WorkRect;                           // Cover the whole scrolling region, shrunk by WindowPadding*1.0f on each side. This is meant to replace ContentRegionRect over time (from 1.71+ onward).
+    ImRect                  WorkRect;                           // Initially covers the whole scrolling region. Reduced by containers e.g columns/tables when active. Shrunk by WindowPadding*1.0f on each side. This is meant to replace ContentRegionRect over time (from 1.71+ onward).
+    ImRect                  ParentWorkRect;                     // Backup of WorkRect before entering a container such as columns/tables. Used by e.g. SpanAllColumns functions to easily access. Stacked containers are responsible for maintaining this. // FIXME-WORKRECT: Could be a stack?
     ImRect                  ClipRect;                           // Current clipping/scissoring rectangle, evolve as we are using PushClipRect(), etc. == DrawList->clip_rect_stack.back().
     ImRect                  ContentRegionRect;                  // FIXME: This is currently confusing/misleading. It is essentially WorkRect but not handling of scrolling. We currently rely on it as right/bottom aligned sizing operation need some size to rely on.
     ImVec2ih                HitTestHoleSize;                    // Define an optional rectangular hole where mouse will pass-through the window.