Fix resizing viewport-owning windows when mouse pos is outside the InnerClipRect (can happen with OS decoration on). (#1542)

imgui.cpp

@@ -5403,9 +5403,12 @@ static bool ImGui::UpdateManualResize(ImGuiWindow* window, const ImVec2& size_au
         ImRect resize_rect(corner - grip.InnerDir * grip_hover_outer_size, corner + grip.InnerDir * grip_hover_inner_size);
         if (resize_rect.Min.x > resize_rect.Max.x) ImSwap(resize_rect.Min.x, resize_rect.Max.x);
         if (resize_rect.Min.y > resize_rect.Max.y) ImSwap(resize_rect.Min.y, resize_rect.Max.y);
+        ImVec2 resize_rect_ref_min = resize_rect.Min;
         resize_rect.ClipWith(clip_viewport_rect);
         bool hovered, held;
         ButtonBehavior(resize_rect, window->GetID(resize_grip_n), &hovered, &held, ImGuiButtonFlags_FlattenChildren | ImGuiButtonFlags_NoNavFocus);
+        if (held && g.ActiveIdIsJustActivated)
+            g.ActiveIdClickOffset = g.IO.MousePos - resize_rect_ref_min; // Override our reference click offset as viewport clipping may be moved it.
         //GetForegroundDrawList(window)->AddRect(resize_rect.Min, resize_rect.Max, IM_COL32(255, 255, 0, 255));
         if (hovered || held)
             g.MouseCursor = (resize_grip_n & 1) ? ImGuiMouseCursor_ResizeNESW : ImGuiMouseCursor_ResizeNWSE;
@@ -5431,8 +5434,11 @@ static bool ImGui::UpdateManualResize(ImGuiWindow* window, const ImVec2& size_au
     {
         bool hovered, held;
         ImRect border_rect = GetResizeBorderRect(window, border_n, grip_hover_inner_size, WINDOWS_RESIZE_FROM_EDGES_HALF_THICKNESS);
+        ImVec2 border_rect_ref_min = border_rect.Min;
         border_rect.ClipWith(clip_viewport_rect);
         ButtonBehavior(border_rect, window->GetID(border_n + 4), &hovered, &held, ImGuiButtonFlags_FlattenChildren);
+        if (held && g.ActiveIdIsJustActivated)
+            g.ActiveIdClickOffset = g.IO.MousePos - border_rect_ref_min; // Override our reference click offset as viewport clipping may be moved it.
         //GetForegroundDrawLists(window)->AddRect(border_rect.Min, border_rect.Max, IM_COL32(255, 255, 0, 255));
         if ((hovered && g.HoveredIdTimer > WINDOWS_RESIZE_FROM_EDGES_FEEDBACK_TIMER) || held)
         {