DragFloat: Fix/workaround for backends which do not preserve a valid mouse position when dragged out of bounds. (#1559)

imgui.cpp

@@ -3580,6 +3580,7 @@ bool ImGui::IsMousePosValid(const ImVec2* mouse_pos)
     return mouse_pos->x >= MOUSE_INVALID && mouse_pos->y >= MOUSE_INVALID;
 }
 
+// NB: This is only valid if IsMousePosValid(). Backends in theory should always keep mouse position valid when dragging even outside the client window.
 ImVec2 ImGui::GetMouseDragDelta(int button, float lock_threshold)
 {
     ImGuiContext& g = *GImGui;
@@ -7622,6 +7623,8 @@ bool ImGui::DragBehavior(const ImRect& frame_bb, ImGuiID id, float* v, float v_s
             float v_cur = g.DragCurrentValue;
             const ImVec2 mouse_drag_delta = GetMouseDragDelta(0, 1.0f);
             float adjust_delta = 0.0f;
+            if (IsMousePosValid())
+            {
                 //if (g.ActiveIdSource == ImGuiInputSource_Mouse)
                 {
                     adjust_delta = mouse_drag_delta.x - g.DragLastMouseDelta.x;
@@ -7630,8 +7633,9 @@ bool ImGui::DragBehavior(const ImRect& frame_bb, ImGuiID id, float* v, float v_s
                     if (g.IO.KeyAlt && g.DragSpeedScaleSlow >= 0.0f)
                         adjust_delta *= g.DragSpeedScaleSlow;
                 }
-            adjust_delta *= v_speed;
                 g.DragLastMouseDelta.x = mouse_drag_delta.x;
+            }
+            adjust_delta *= v_speed;
 
             if (fabsf(adjust_delta) > 0.0f)
             {