Made it possible to move a window by dragging from the Collapse button (past the drag threshold). Extracted some code out to a CollapseButton() function.

imgui.cpp

@@ -305,7 +305,7 @@
  When you are not sure about a old symbol or function name, try using the Search/Find function of your IDE to look for comments or references in all imgui files.
  You can read releases logs https://github.com/ocornut/imgui/releases for more details.
 
- - 2017/07/22 (1.63) - changed ImGui::GetTime() return value from float to double to avoid accumulating floating point imprecisions over time.
+ - 2018/07/22 (1.63) - changed ImGui::GetTime() return value from float to double to avoid accumulating floating point imprecisions over time.
  - 2018/07/08 (1.63) - style: renamed ImGuiCol_ModalWindowDarkening to ImGuiCol_ModalWindowDimBg for consistency with other features. Kept redirection enum (will obsolete).
  - 2018/07/06 (1.63) - removed per-window ImGuiWindowFlags_ResizeFromAnySide beta flag in favor of a global io.OptResizeWindowsFromEdges to enable the feature.
  - 2018/06/06 (1.62) - renamed GetGlyphRangesChinese() to GetGlyphRangesChineseFull() to distinguish other variants and discourage using the full set.
@@ -6655,15 +6655,8 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
 
             // Collapse button
             if (!(flags & ImGuiWindowFlags_NoCollapse))
-            {
+                if (CollapseButton(window->GetID("#COLLAPSE"), window->Pos + style.FramePadding))
-                ImGuiID id = window->GetID("#COLLAPSE");
-                ImRect bb(window->Pos + style.FramePadding + ImVec2(1,1), window->Pos + style.FramePadding + ImVec2(g.FontSize,g.FontSize) - ImVec2(1,1));
-                ItemAdd(bb, id);
-                if (ButtonBehavior(bb, id, NULL, NULL))
                     window->CollapseToggleWanted = true; // Defer collapsing to next frame as we are too far in the Begin() function
-                RenderNavHighlight(bb, id);
-                RenderArrow(window->Pos + style.FramePadding, window->Collapsed ? ImGuiDir_Right : ImGuiDir_Down, 1.0f);
-            }
 
             // Close button
             if (p_open != NULL)
@@ -8336,6 +8329,24 @@ bool ImGui::CloseButton(ImGuiID id, const ImVec2& pos, float radius)
     return pressed;
 }
 
+bool ImGui::CollapseButton(ImGuiID id, const ImVec2& pos)
+{
+    ImGuiContext& g = *GImGui;
+    ImGuiWindow* window = g.CurrentWindow;
+
+    ImRect bb(pos, pos + ImVec2(g.FontSize, g.FontSize));
+    ItemAdd(bb, id);
+    bool ret = ButtonBehavior(bb, id, NULL, NULL, ImGuiButtonFlags_None);
+    RenderNavHighlight(bb, id);
+    RenderArrow(bb.Min, window->Collapsed ? ImGuiDir_Right : ImGuiDir_Down, 1.0f);
+
+    // Switch to moving the window after mouse is moved beyond the initial drag threshold
+    if (IsItemActive() && IsMouseDragging())
+        StartMouseMovingWindow(window);
+
+    return ret;
+}
+
 void ImGui::Image(ImTextureID user_texture_id, const ImVec2& size, const ImVec2& uv0, const ImVec2& uv1, const ImVec4& tint_col, const ImVec4& border_col)
 {
     ImGuiWindow* window = GetCurrentWindow();

imgui_internal.h

@@ -1172,6 +1172,7 @@ namespace ImGui
     IMGUI_API bool          ButtonBehavior(const ImRect& bb, ImGuiID id, bool* out_hovered, bool* out_held, ImGuiButtonFlags flags = 0);
     IMGUI_API bool          ButtonEx(const char* label, const ImVec2& size_arg = ImVec2(0,0), ImGuiButtonFlags flags = 0);
     IMGUI_API bool          CloseButton(ImGuiID id, const ImVec2& pos, float radius);
+    IMGUI_API bool          CollapseButton(ImGuiID id, const ImVec2& pos);
     IMGUI_API bool          ArrowButtonEx(const char* str_id, ImGuiDir dir, ImVec2 size_arg, ImGuiButtonFlags flags);
 
     IMGUI_API bool          DragBehavior(ImGuiID id, ImGuiDataType data_type, void* v, float v_speed, const void* v_min, const void* v_max, const char* format, float power);