Tables: fixed calculation of multi-instance shared decoration/scrollbar width of scrolling tables. (#5920, #6619)

Avoid width variation when resizing down a table hosting a child window.
+ shallow tweak to GetContentRegionMax().

docs/CHANGELOG.txt

@@ -64,6 +64,9 @@ Other changes:
 - InputText: Fixed a case where deactivation frame would write to underlying
   buffer or call CallbackResize although unnecessary, in a frame where the
   return value was false.
+- Tables: fixed calculation of multi-instance shared decoration/scrollbar width of
+  scrolling tables, to avoid flickering width variation when resizing down a table
+  hosting a child window. (#5920, #6619)
 - Scrollbar: layout needs to take account of window border size, so a border size
   will slightly reduce scrollbar size. Generally we tried to make it that window
   border size has no incidence on layout but this can't work with thick borders. (#2522)

imgui.cpp

@@ -6738,6 +6738,7 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
 
         // [LEGACY] Content Region
         // FIXME-OBSOLETE: window->ContentRegionRect.Max is currently very misleading / partly faulty, but some BeginChild() patterns relies on it.
+        // Unless explicit content size is specified by user, this currently represent the region leading to no scrolling.
         // Used by:
         // - Mouse wheel scrolling + many other things
         window->ContentRegionRect.Min.x = window->Pos.x - window->Scroll.x + window->WindowPadding.x + window->DecoOuterSizeX1;
@@ -9773,10 +9774,10 @@ ImVec2 ImGui::GetContentRegionMax()
 {
     ImGuiContext& g = *GImGui;
     ImGuiWindow* window = g.CurrentWindow;
-    ImVec2 mx = window->ContentRegionRect.Max - window->Pos;
+    ImVec2 mx = window->ContentRegionRect.Max;
     if (window->DC.CurrentColumns || g.CurrentTable)
-        mx.x = window->WorkRect.Max.x - window->Pos.x;
-    return mx;
+        mx.x = window->WorkRect.Max.x;
+    return mx - window->Pos;
 }
 
 // [Internal] Absolute coordinate. Saner. This is not exposed until we finishing refactoring work rect features.

imgui.h

@@ -26,7 +26,7 @@
 // Library Version
 // (Integer encoded as XYYZZ for use in #if preprocessor conditionals, e.g. '#if IMGUI_VERSION_NUM >= 12345')
 #define IMGUI_VERSION       "1.89.8 WIP"
-#define IMGUI_VERSION_NUM   18973
+#define IMGUI_VERSION_NUM   18974
 #define IMGUI_HAS_TABLE
 
 /*

imgui_tables.cpp

@@ -410,7 +410,7 @@ bool    ImGui::BeginTableEx(const char* name, ImGuiID id, int columns_count, ImG
             table->HasScrollbarYPrev = table->HasScrollbarYCurr;
             table->HasScrollbarYCurr = false;
         }
-        table->HasScrollbarYCurr |= (table->InnerWindow->ScrollMax.y > 0.0f);
+        table->HasScrollbarYCurr |= table->InnerWindow->ScrollbarY;
     }
     else
     {