Drag and Drop: can use BeginDragDropSource() for other than the left mouse button (#1637, #3885)

As long as the item has an ID (for ID-less items will add new functionalities later.
Amend 2c3c5125

docs/CHANGELOG.txt

@@ -49,7 +49,7 @@ Other Changes:
 - Window: Shrink close button hit-testing region when it covers an abnormally high portion of the window visible
   area (e.g. when window is collapsed + moved in a corner) to facilitate moving the window away. (#3825)
 - Window, Nav: Fixed crash when calling SetWindowFocus(NULL) as the time a new window appears. (#3865) [@nem0]
-- DragScalar: Fixed crash when using DragScalar() directly (not via common wrapper like DragFloat() etc.)
+- Drags: Fixed crash when using DragScalar() directly (not via common wrapper like DragFloat() etc.)
   with ImGuiSliderFlags_AlwaysClamp + only one of either p_min or p_max set. (#3824) [@harry75369]
 - Drags, Sliders: Fixed a bug where editing value would use wrong number if there were digits right after 
   format specifier (e.g. using "%f123" as a format string). [@rokups]
@@ -59,6 +59,8 @@ Other Changes:
 - IsItemHovered(): fixed return value false positive when used after EndChild(), EndGroup() or widgets using
   either of them, when the hovered location is located within a child window, e.g. InputTextMultiline().
   This is intended to have no side effects, but brace yourself for the possible comeback.. (#3851, #1370)
+- Drag and Drop: can use BeginDragDropSource() for other than the left mouse button as long as the item 
+  has an ID (for ID-less items will add new functionalities later). (#1637, #3885)
 - Added GetAllocatorFunctions() to facilitate sharing allocators accross DLL boundaries. (#3836)
 - ImFontAtlas: Added 'bool TexPixelsUseColors' output to help backend decide of underlying texture format. (#3369)
   This can currently only ever be set by the Freetype renderer.

imgui.cpp

@@ -3024,6 +3024,7 @@ void ImGui::SetActiveID(ImGuiID id, ImGuiWindow* window)
         g.ActiveIdTimer = 0.0f;
         g.ActiveIdHasBeenPressedBefore = false;
         g.ActiveIdHasBeenEditedBefore = false;
+        g.ActiveIdMouseButton = -1;
         if (id != 0)
         {
             g.LastActiveId = id;
@@ -9643,27 +9644,45 @@ void ImGui::ClearDragDrop()
     memset(&g.DragDropPayloadBufLocal, 0, sizeof(g.DragDropPayloadBufLocal));
 }
 
-// Call when current ID is active.
 // When this returns true you need to: a) call SetDragDropPayload() exactly once, b) you may render the payload visual/description, c) call EndDragDropSource()
+// If the item has an identifier:
+// - This assume/require the item to be activated (typically via ButtonBehavior).
+// - Therefore if you want to use this with a mouse button other than left mouse button, it is up to the item itself to activate with another button.
+// - We then pull and use the mouse button that was used to activate the item and use it to carry on the drag.
+// If the item has no identifier:
+// - Currently always assume left mouse button.
 bool ImGui::BeginDragDropSource(ImGuiDragDropFlags flags)
 {
     ImGuiContext& g = *GImGui;
     ImGuiWindow* window = g.CurrentWindow;
 
+    // FIXME-DRAGDROP: While in the common-most "drag from non-zero active id" case we can tell the mouse button,
+    // in both SourceExtern and id==0 cases we may requires something else (explicit flags or some heuristic).
+    ImGuiMouseButton mouse_button = ImGuiMouseButton_Left;
+
     bool source_drag_active = false;
     ImGuiID source_id = 0;
     ImGuiID source_parent_id = 0;
-    ImGuiMouseButton mouse_button = ImGuiMouseButton_Left;
     if (!(flags & ImGuiDragDropFlags_SourceExtern))
     {
         source_id = window->DC.LastItemId;
-        if (source_id != 0 && g.ActiveId != source_id) // Early out for most common case
-            return false;
-        if (g.IO.MouseDown[mouse_button] == false)
-            return false;
-
-        if (source_id == 0)
+        if (source_id != 0)
+        {
+            // Common path: items with ID
+            if (g.ActiveId != source_id)
+                return false;
+            if (g.ActiveIdMouseButton != -1)
+                mouse_button = g.ActiveIdMouseButton;
+            if (g.IO.MouseDown[mouse_button] == false)
+                return false;
+            g.ActiveIdAllowOverlap = false;
+        }
+        else
         {
+            // Uncommon path: items without ID
+            if (g.IO.MouseDown[mouse_button] == false)
+                return false;
+
             // If you want to use BeginDragDropSource() on an item with no unique identifier for interaction, such as Text() or Image(), you need to:
             // A) Read the explanation below, B) Use the ImGuiDragDropFlags_SourceAllowNullID flag, C) Swallow your programmer pride.
             if (!(flags & ImGuiDragDropFlags_SourceAllowNullID))
@@ -9690,10 +9709,6 @@ bool ImGui::BeginDragDropSource(ImGuiDragDropFlags flags)
             if (g.ActiveId == source_id) // Allow the underlying widget to display/return hovered during the mouse release frame, else we would get a flicker.
                 g.ActiveIdAllowOverlap = is_hovered;
         }
-        else
-        {
-            g.ActiveIdAllowOverlap = false;
-        }
         if (g.ActiveId != source_id)
             return false;
         source_parent_id = window->IDStack.back();
@@ -9896,7 +9911,7 @@ const ImGuiPayload* ImGui::AcceptDragDropPayload(const char* type, ImGuiDragDrop
     flags |= (g.DragDropSourceFlags & ImGuiDragDropFlags_AcceptNoDrawDefaultRect); // Source can also inhibit the preview (useful for external sources that lives for 1 frame)
     if (!(flags & ImGuiDragDropFlags_AcceptNoDrawDefaultRect) && payload.Preview)
     {
-        // FIXME-DRAG: Settle on a proper default visuals for drop target.
+        // FIXME-DRAGDROP: Settle on a proper default visuals for drop target.
         r.Expand(3.5f);
         bool push_clip_rect = !window->ClipRect.Contains(r);
         if (push_clip_rect) window->DrawList->PushClipRect(r.Min - ImVec2(1, 1), r.Max + ImVec2(1, 1));

imgui.h

@@ -755,8 +755,11 @@ namespace ImGui
     IMGUI_API void          LogTextV(const char* fmt, va_list args) IM_FMTLIST(1);
 
     // Drag and Drop
-    // - If you stop calling BeginDragDropSource() the payload is preserved however it won't have a preview tooltip (we currently display a fallback "..." tooltip as replacement)
-    IMGUI_API bool          BeginDragDropSource(ImGuiDragDropFlags flags = 0);                                      // call when the current item is active. If this return true, you can call SetDragDropPayload() + EndDragDropSource()
+    // - On source items, call BeginDragDropSource(), if it returns true also call SetDragDropPayload() + EndDragDropSource().
+    // - On target candidates, call BeginDragDropTarget(), if it returns true also call AcceptDragDropPayload() + EndDragDropTarget().
+    // - If you stop calling BeginDragDropSource() the payload is preserved however it won't have a preview tooltip (we currently display a fallback "..." tooltip, see #1725)
+    // - An item can be both drag source and drop target.
+    IMGUI_API bool          BeginDragDropSource(ImGuiDragDropFlags flags = 0);                                      // call after submitting an item which may be dragged. when this return true, you can call SetDragDropPayload() + EndDragDropSource()
     IMGUI_API bool          SetDragDropPayload(const char* type, const void* data, size_t sz, ImGuiCond cond = 0);  // type is a user defined string of maximum 32 characters. Strings starting with '_' are reserved for dear imgui internal types. Data is copied and held by imgui.
     IMGUI_API void          EndDragDropSource();                                                                    // only call EndDragDropSource() if BeginDragDropSource() returns true!
     IMGUI_API bool                  BeginDragDropTarget();                                                          // call after submitting an item that may receive a payload. If this returns true, you can call AcceptDragDropPayload() + EndDragDropTarget()

imgui_internal.h

@@ -1568,7 +1568,7 @@ struct ImGuiContext
         ActiveIdClickOffset = ImVec2(-1, -1);
         ActiveIdWindow = NULL;
         ActiveIdSource = ImGuiInputSource_None;
-        ActiveIdMouseButton = 0;
+        ActiveIdMouseButton = -1;
         ActiveIdPreviousFrame = 0;
         ActiveIdPreviousFrameIsAlive = false;
         ActiveIdPreviousFrameHasBeenEditedBefore = false;