Tables: Clear scroll value when reactivating scroll. Assert when passing width/weight to TableSetupColumn() without an explicit sizing policy at either table or column label.

Clearing scroll was initially made desirable by tests exhibiting side effects but generally feels saner.
table->UserOuterSize is poorly used in this commit but will be by subsequent commits.

imgui_internal.h

@@ -2037,6 +2037,7 @@ struct ImGuiTable
     ImVec2                      HostBackupPrevLineSize;     // Backup of InnerWindow->DC.PrevLineSize at the end of BeginTable()
     ImVec2                      HostBackupCurrLineSize;     // Backup of InnerWindow->DC.CurrLineSize at the end of BeginTable()
     ImVec2                      HostBackupCursorMaxPos;     // Backup of InnerWindow->DC.CursorMaxPos at the end of BeginTable()
+    ImVec2                      UserOuterSize;              // outer_size.x passed to BeginTable()
     ImVec1                      HostBackupColumnsOffset;    // Backup of OuterWindow->DC.ColumnsOffset at the end of BeginTable()
     float                       HostBackupItemWidth;        // Backup of OuterWindow->DC.ItemWidth at the end of BeginTable()
     int                         HostBackupItemWidthStackSize;// Backup of OuterWindow->DC.ItemWidthStack.Size at the end of BeginTable()
@@ -2084,6 +2085,7 @@ struct ImGuiTable
     bool                        IsResetDisplayOrderRequest;
     bool                        IsUnfrozenRows;             // Set when we got past the frozen row.
     bool                        IsOuterRectMinFitX;         // Set when outer_size.x == 0.0f in BeginTable(), scrolling is disabled, and there are no stretch columns.
+    bool                        IsDefaultSizingPolicy;      // Set if user didn't explicitely set a sizing policy in BeginTable()
     bool                        MemoryCompacted;
     bool                        HostSkipItems;              // Backup of InnerWindow->SkipItem at the end of BeginTable(), because we will overwrite InnerWindow->SkipItem on a per-column basis
 

imgui_tables.cpp

@@ -324,6 +324,7 @@ bool    ImGui::BeginTableEx(const char* name, ImGuiID id, int columns_count, ImG
         IM_ASSERT(table->ColumnsCount == columns_count && "BeginTable(): Cannot change columns count mid-frame while preserving same ID");
 
     // Fix flags
+    table->IsDefaultSizingPolicy = (flags & ImGuiTableFlags_SizingMask_) == 0;
     flags = TableFixFlags(flags, outer_window);
 
     // Initialize
@@ -335,7 +336,7 @@ bool    ImGui::BeginTableEx(const char* name, ImGuiID id, int columns_count, ImG
     table->ColumnsCount = columns_count;
     table->IsLayoutLocked = false;
     table->InnerWidth = inner_width;
-    table->IsOuterRectMinFitX = (outer_size.x == 0.0f) && (use_child_window == false); // Will be set to false later if there are any Stretch column.
+    table->UserOuterSize = outer_size;
 
     // When not using a child window, WorkRect.Max will grow as we append contents.
     if (use_child_window)
@@ -356,6 +357,10 @@ bool    ImGui::BeginTableEx(const char* name, ImGuiID id, int columns_count, ImG
         if (override_content_size.x != FLT_MAX || override_content_size.y != FLT_MAX)
             SetNextWindowContentSize(ImVec2(override_content_size.x != FLT_MAX ? override_content_size.x : 0.0f, override_content_size.y != FLT_MAX ? override_content_size.y : 0.0f));
 
+        // Reset scroll if we are reactivating it
+        if ((table_last_flags & (ImGuiTableFlags_ScrollX | ImGuiTableFlags_ScrollY)) == 0)
+            SetNextWindowScroll(ImVec2(0.0f, 0.0f));
+
         // Create scrolling region (without border and zero window padding)
         ImGuiWindowFlags child_flags = (flags & ImGuiTableFlags_ScrollX) ? ImGuiWindowFlags_HorizontalScrollbar : ImGuiWindowFlags_None;
         BeginChildEx(name, instance_id, outer_rect.GetSize(), false, child_flags);
@@ -1031,8 +1036,7 @@ void ImGui::TableUpdateLayout(ImGuiTable* table)
     // [Part 8] Lock actual OuterRect/WorkRect right-most position.
     // This is done late to handle the case of fixed-columns tables not claiming more widths that they need.
     // Because of this we are careful with uses of WorkRect and InnerClipRect before this point.
-    if (table->RightMostStretchedColumn != -1)
-        table->IsOuterRectMinFitX = false;
+    table->IsOuterRectMinFitX = (table->UserOuterSize.x == 0.0f) && table->RightMostStretchedColumn == -1 && (table->InnerWindow == table->OuterWindow);
     if (table->IsOuterRectMinFitX)
     {
         table->OuterRect.Max.x = table->WorkRect.Max.x = unused_x1;
@@ -1338,6 +1342,11 @@ void ImGui::TableSetupColumn(const char* label, ImGuiTableColumnFlags flags, flo
     ImGuiTableColumn* column = &table->Columns[table->DeclColumnsCount];
     table->DeclColumnsCount++;
 
+    // Assert when passing a width or weight if policy is entirely left to default, to avoid storing width into weight and vice-versa.
+    // Give a grace to users of ImGuiTableFlags_ScrollX.
+    if (table->IsDefaultSizingPolicy && (flags & ImGuiTableColumnFlags_WidthMask_) == 0 && (flags & ImGuiTableFlags_ScrollX) == 0)
+        IM_ASSERT(init_width_or_weight <= 0.0f && "Can only specify width/weight if sizing policy is set explicitely in either Table or Column.");
+
     // When passing a width automatically enforce WidthFixed policy
     // (whereas TableSetupColumnFlags would default to WidthAuto if table is not Resizable)
     if ((flags & ImGuiTableColumnFlags_WidthMask_) == 0 && init_width_or_weight > 0.0f)