Resizing window doesn't rely on MouseDelta anymore, but rather recompute expected size based absolute mouse coords. (#668)

Storing ActiveIdClickOffset to generalize pattern already used by
columns.

imgui.cpp

@@ -3993,7 +3993,7 @@ bool ImGui::Begin(const char* name, bool* p_open, const ImVec2& size_on_first_us
         if ((flags & ImGuiWindowFlags_ChildWindow) && !(flags & ImGuiWindowFlags_Popup))
         {
             window->Pos = window->PosFloat = parent_window->DC.CursorPos;
-            window->Size = window->SizeFull = size_on_first_use; // NB: argument name 'size_on_first_use' misleading here, it's really just 'size' as provided by user.
+            window->Size = window->SizeFull = size_on_first_use; // NB: argument name 'size_on_first_use' misleading here, it's really just 'size' as provided by user passed via BeginChild()->Begin().
         }
 
         bool window_pos_center = false;
@@ -4108,7 +4108,8 @@ bool ImGui::Begin(const char* name, bool* p_open, const ImVec2& size_on_first_us
                 }
                 else if (held)
                 {
-                    window->SizeFull = ImMax(window->SizeFull + g.IO.MouseDelta, style.WindowMinSize);
+                    // We don't use an incremental MouseDelta but rather compute an absolute target size based on mouse position
+                    window->SizeFull = ImMax((g.IO.MousePos - g.ActiveIdClickOffset + resize_rect.GetSize()) - window->Pos, style.WindowMinSize);
                     if (!(flags & ImGuiWindowFlags_NoSavedSettings))
                         MarkSettingsDirty();
                 }
@@ -5423,6 +5424,7 @@ bool ImGui::ButtonBehavior(const ImRect& bb, ImGuiID id, bool* out_hovered, bool
             {
                 SetActiveID(id, window); // Hold on ID
                 FocusWindow(window);
+                g.ActiveIdClickOffset = g.IO.MousePos - bb.Min;
             }
             if (((flags & ImGuiButtonFlags_PressedOnClick) && g.IO.MouseClicked[0]) || ((flags & ImGuiButtonFlags_PressedOnDoubleClick) && g.IO.MouseDoubleClicked[0]))
             {
@@ -9188,7 +9190,7 @@ static float GetDraggedColumnOffset(int column_index)
     IM_ASSERT(column_index > 0); // We cannot drag column 0. If you get this assert you may have a conflict between the ID of your columns and another widgets.
     IM_ASSERT(g.ActiveId == window->DC.ColumnsSetID + ImGuiID(column_index));
 
-    float x = g.IO.MousePos.x + g.ActiveClickDeltaToCenter.x - window->Pos.x;
+    float x = g.IO.MousePos.x - g.ActiveIdClickOffset.x - window->Pos.x;
     x = ImClamp(x, ImGui::GetColumnOffset(column_index-1)+g.Style.ColumnsMinSpacing, ImGui::GetColumnOffset(column_index+1)-g.Style.ColumnsMinSpacing);
 
     return (float)(int)x;
@@ -9293,7 +9295,7 @@ void ImGui::Columns(int columns_count, const char* id, bool border)
             if (held)
             {
                 if (g.ActiveIdIsJustActivated)
-                    g.ActiveClickDeltaToCenter.x = x - g.IO.MousePos.x;
+                    g.ActiveIdClickOffset.x -= 4;   // Store from center of column line
                 x = GetDraggedColumnOffset(i);
                 SetColumnOffset(i, x);
             }

imgui_demo.cpp

@@ -1807,10 +1807,8 @@ static void ShowExampleAppFixedOverlay(bool* p_open)
     ImGui::End();
 }
 
-static void ShowExampleAppManipulatingWindowTitle(bool* p_open)
+static void ShowExampleAppManipulatingWindowTitle(bool*)
 {
-    (void)p_open;
-
     // By default, Windows are uniquely identified by their title.
     // You can use the "##" and "###" markers to manipulate the display/ID. Read FAQ at the top of this file!
 

imgui_internal.h

@@ -373,6 +373,7 @@ struct ImGuiContext
     bool                    ActiveIdIsAlive;
     bool                    ActiveIdIsJustActivated;            // Set at the time of activation for one frame
     bool                    ActiveIdAllowOverlap;               // Set only by active widget
+    ImVec2                  ActiveIdClickOffset;                // Clicked offset from upper-left corner, if applicable (currently only set by ButtonBehavior)
     ImGuiWindow*            ActiveIdWindow;
     ImGuiWindow*            MovedWindow;                        // Track the child window we clicked on to move a window.
     ImGuiID                 MovedWindowMoveId;                  // == MovedWindow->RootWindow->MoveId
@@ -410,7 +411,6 @@ struct ImGuiContext
     ImFont                  InputTextPasswordFont;
     ImGuiID                 ScalarAsInputTextId;                // Temporary text input when CTRL+clicking on a slider, etc.
     ImGuiStorage            ColorEditModeStorage;               // Store user selection of color edit mode
-    ImVec2                  ActiveClickDeltaToCenter;
     float                   DragCurrentValue;                   // Currently dragged value, always float, not rounded by end-user precision settings
     ImVec2                  DragLastMouseDelta;
     float                   DragSpeedDefaultRatio;              // If speed == 0.0f, uses (max-min) * DragSpeedDefaultRatio
@@ -458,6 +458,7 @@ struct ImGuiContext
         ActiveIdIsAlive = false;
         ActiveIdIsJustActivated = false;
         ActiveIdAllowOverlap = false;
+        ActiveIdClickOffset = ImVec2(-1,-1);
         ActiveIdWindow = NULL;
         MovedWindow = NULL;
         MovedWindowMoveId = 0;
@@ -475,7 +476,6 @@ struct ImGuiContext
         SetNextTreeNodeOpenCond = 0;
 
         ScalarAsInputTextId = 0;
-        ActiveClickDeltaToCenter = ImVec2(0.0f, 0.0f);
         DragCurrentValue = 0.0f;
         DragLastMouseDelta = ImVec2(0.0f, 0.0f);
         DragSpeedDefaultRatio = 0.01f;