Windows: store BgClickFlags which allows the equivalent of io.ConfigWindowsMoveFromTitleBarOnly to be overridden on a per window basis. (#899, #3071, #5044, + #3379)

io.ConfigWindowsMoveFromTitleBarOnly now sets initial value for BgClickFlags. Using e.g. ImGui::GetCurrentWindow()->BgClickFlags &= ~ImGuiWindowBgClickFlags_Move; allow per-window override.
This will be extended for supporting scrolling options for #3379.
As a minor side effect: the effect of enabling io.ConfigWindowsMoveFromTitleBarOnly now happens one frame later ('window_modal_bounds_exceeding_work_area" test accidentally broke in some situations because of that)

imgui.cpp

@@ -5217,7 +5217,7 @@ void ImGui::UpdateMouseMovingWindowEndFrame()
             // Please note how StartMouseMovingWindow() and StopMouseMovingWindow() and not entirely symetrical, at the later doesn't clear ActiveId.
 
             // Cancel moving if clicked outside of title bar
-            if (g.IO.ConfigWindowsMoveFromTitleBarOnly)
+            if ((hovered_window->BgClickFlags & ImGuiWindowBgClickFlags_Move) == 0) // set by io.ConfigWindowsMoveFromTitleBarOnly
                 if (!(hovered_root->Flags & ImGuiWindowFlags_NoTitleBar))
                     if (!hovered_root->TitleBarRect().Contains(g.IO.MouseClickedPos[0]))
                         g.MovingWindow = NULL;
@@ -6768,7 +6768,7 @@ static int ImGui::UpdateWindowManualResize(ImGuiWindow* window, const ImVec2& si
     const float grip_hover_outer_size = g.WindowsBorderHoverPadding;
 
     ImRect clamp_rect = visibility_rect;
-    const bool window_move_from_title_bar = g.IO.ConfigWindowsMoveFromTitleBarOnly && !(window->Flags & ImGuiWindowFlags_NoTitleBar);
+    const bool window_move_from_title_bar = !(window->BgClickFlags & ImGuiWindowBgClickFlags_Move) && !(window->Flags & ImGuiWindowFlags_NoTitleBar);
     if (window_move_from_title_bar)
         clamp_rect.Min.y -= window->TitleBarHeight;
 
@@ -6957,9 +6957,8 @@ static int ImGui::UpdateWindowManualResize(ImGuiWindow* window, const ImVec2& si
 
 static inline void ClampWindowPos(ImGuiWindow* window, const ImRect& visibility_rect)
 {
-    ImGuiContext& g = *GImGui;
     ImVec2 size_for_clamping = window->Size;
-    if (g.IO.ConfigWindowsMoveFromTitleBarOnly && !(window->Flags & ImGuiWindowFlags_NoTitleBar))
+    if (!(window->BgClickFlags & ImGuiWindowBgClickFlags_Move) && !(window->Flags & ImGuiWindowFlags_NoTitleBar))
         size_for_clamping.y = window->TitleBarHeight;
     window->Pos = ImClamp(window->Pos, visibility_rect.Min - size_for_clamping, visibility_rect.Max);
 }
@@ -7928,6 +7927,11 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         if (flags & ImGuiWindowFlags_Tooltip)
             g.TooltipPreviousWindow = window;
 
+        // Set default BgClickFlags
+        // This is set at the end of this function, so UpdateManualResize()/ClampWindowPos() may use last-frame value if overriden by user code.
+        // FIXME: The general intent is that we will later expose config options to default to enable scrolling + select scrolling mouse button.
+        window->BgClickFlags = g.IO.ConfigWindowsMoveFromTitleBarOnly ? ImGuiWindowBgClickFlags_None : ImGuiWindowBgClickFlags_Move;
+
         // We fill last item data based on Title Bar/Tab, in order for IsItemHovered() and IsItemActive() to be usable after Begin().
         // This is useful to allow creating context menus on title bar only, etc.
         window->DC.WindowItemStatusFlags = ImGuiItemStatusFlags_None;

imgui_internal.h

@@ -209,6 +209,7 @@ typedef int ImGuiSeparatorFlags;        // -> enum ImGuiSeparatorFlags_     // F
 typedef int ImGuiTextFlags;             // -> enum ImGuiTextFlags_          // Flags: for TextEx()
 typedef int ImGuiTooltipFlags;          // -> enum ImGuiTooltipFlags_       // Flags: for BeginTooltipEx()
 typedef int ImGuiTypingSelectFlags;     // -> enum ImGuiTypingSelectFlags_  // Flags: for GetTypingSelectRequest()
+typedef int ImGuiWindowBgClickFlags;    // -> enum ImGuiWindowBgClickFlags_ // Flags: for overriding behavior of clicking on window background/void.
 typedef int ImGuiWindowRefreshFlags;    // -> enum ImGuiWindowRefreshFlags_ // Flags: for SetNextWindowRefreshPolicy()
 
 // Table column indexing
@@ -1283,6 +1284,12 @@ enum ImGuiWindowRefreshFlags_
     // Refresh policy/frequency, Load Balancing etc.
 };
 
+enum ImGuiWindowBgClickFlags_
+{
+    ImGuiWindowBgClickFlags_None                = 0,
+    ImGuiWindowBgClickFlags_Move                = 1 << 0,   // Click on bg/void + drag to move window. Cleared by default when using io.ConfigWindowsMoveFromTitleBarOnly.
+};
+
 enum ImGuiNextWindowDataFlags_
 {
     ImGuiNextWindowDataFlags_None               = 0,
@@ -2662,6 +2669,7 @@ struct IMGUI_API ImGuiWindow
     ImS8                    HiddenFramesCannotSkipItems;        // Hide the window for N frames while allowing items to be submitted so we can measure their size
     ImS8                    HiddenFramesForRenderOnly;          // Hide the window until frame N at Render() time only
     ImS8                    DisableInputsFrames;                // Disable window interactions for N frames
+    ImGuiWindowBgClickFlags BgClickFlags : 8;                   // Configure behavior of click+dragging on window bg/void or over items. Default sets by io.ConfigWindowsMoveFromTitleBarOnly. If you use this please report in #3379.
     ImGuiCond               SetWindowPosAllowFlags : 8;         // store acceptable condition flags for SetNextWindowPos() use.
     ImGuiCond               SetWindowSizeAllowFlags : 8;        // store acceptable condition flags for SetNextWindowSize() use.
     ImGuiCond               SetWindowCollapsedAllowFlags : 8;   // store acceptable condition flags for SetNextWindowCollapsed() use.