Backends: SDL: Fix multi-viewport dragging issue with SDL on some systems. (v2 for master, using bd->MouseButtonsDown == 0) (#5012, #5082)

# Conflicts:
#	backends/imgui_impl_sdl.cpp
#	docs/CHANGELOG.txt

backends/imgui_impl_sdl.cpp

@@ -18,6 +18,7 @@
 
 // CHANGELOG
 // (minor and older changes stripped away, please see git history for details)
+//  2022-03-22: Inputs: Fix mouse position issues when dragging outside of boundaries. SDL_CaptureMouse() erroneously still gives out LEAVE events when hovering OS decorations.
 //  2022-03-22: Inputs: Added support for extra mouse buttons (SDL_BUTTON_X1/SDL_BUTTON_X2).
 //  2022-02-04: Added SDL_Renderer* parameter to ImGui_ImplSDL2_InitForSDLRenderer(), so we can use SDL_GetRendererOutputSize() instead of SDL_GL_GetDrawableSize() when bound to a SDL_Renderer.
 //  2022-01-26: Inputs: replaced short-lived io.AddKeyModsEvent() (added two weeks ago) with io.AddKeyEvent() using ImGuiKey_ModXXX flags. Sorry for the confusion.
@@ -85,6 +86,7 @@ struct ImGui_ImplSDL2_Data
     Uint64          Time;
     int             MouseButtonsDown;
     SDL_Cursor*     MouseCursors[ImGuiMouseCursor_COUNT];
+    int             PendingMouseLeaveFrame;
     char*           ClipboardTextData;
     bool            MouseCanUseGlobalState;
 
@@ -292,9 +294,17 @@ bool ImGui_ImplSDL2_ProcessEvent(const SDL_Event* event)
         }
         case SDL_WINDOWEVENT:
         {
-            if (event->window.event == SDL_WINDOWEVENT_LEAVE)
-                io.AddMousePosEvent(-FLT_MAX, -FLT_MAX);
-            if (event->window.event == SDL_WINDOWEVENT_FOCUS_GAINED)
+            // - When capturing mouse, SDL will send a bunch of conflicting LEAVE/ENTER event on every mouse move, but the final ENTER tends to be right.
+            // - However we won't get a correct LEAVE event for a captured window.
+            // - In some cases, when detaching a window from main viewport SDL may send SDL_WINDOWEVENT_ENTER one frame too late,
+            //   causing SDL_WINDOWEVENT_LEAVE on previous frame to interrupt drag operation by clear mouse position. This is why
+            //   we delay process the SDL_WINDOWEVENT_LEAVE events by one frame. See issue #5012 for details.
+            Uint8 window_event = event->window.event;
+            if (window_event == SDL_WINDOWEVENT_ENTER)
+                bd->PendingMouseLeaveFrame = 0;
+            if (window_event == SDL_WINDOWEVENT_LEAVE)
+                bd->PendingMouseLeaveFrame = ImGui::GetFrameCount() + 1;
+            if (window_event == SDL_WINDOWEVENT_FOCUS_GAINED)
                 io.AddFocusEvent(true);
             else if (event->window.event == SDL_WINDOWEVENT_FOCUS_LOST)
                 io.AddFocusEvent(false);
@@ -540,6 +550,12 @@ void ImGui_ImplSDL2_NewFrame()
     io.DeltaTime = bd->Time > 0 ? (float)((double)(current_time - bd->Time) / frequency) : (float)(1.0f / 60.0f);
     bd->Time = current_time;
 
+    if (bd->PendingMouseLeaveFrame && bd->PendingMouseLeaveFrame >= ImGui::GetFrameCount() && bd->MouseButtonsDown == 0)
+    {
+        io.AddMousePosEvent(-FLT_MAX, -FLT_MAX);
+        bd->PendingMouseLeaveFrame = 0;
+    }
+
     ImGui_ImplSDL2_UpdateMouseData();
     ImGui_ImplSDL2_UpdateMouseCursor();
 

docs/CHANGELOG.txt

@@ -69,6 +69,7 @@ Other Changes:
 - Misc: Updated stb_rect_pack.h from 1.00 to 1.01 (minor). (#5075)
 - Misc: binary_to_compressed_c tool: Added -nostatic option. (#5021) [@podsvirov]
 - ImVector: Fixed erase() with empty range. (#5009) [@thedmd]
+- Backends: SDL: Fixed dragging out viewport broken on some SDL setups. (#5012) [@rokups]
 - Backends: SDL: Added support for extra mouse buttons (SDL_BUTTON_X1/SDL_BUTTON_X2). (#5125) [@sgiurgiu]
 - Examples: Emscripten: Fix building for latest Emscripten specs. (#3632)