Docking: Remove code in BeginDocked() to set HiddenFramesCannotSkipItems based on upcoming tab bar selection, solely based on focus (might break something subtle?). Follow-up to c355ed126775f36dfb19202af8526ba2ce183877. (#2453, #2109)

imgui.cpp

@@ -6070,9 +6070,13 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
     window->BeginCount++;
     g.NextWindowData.Clear();
 
+    // When we are about to select this tab (which will only be visible on the _next frame_), flag it with a non-zero HiddenFramesCannotSkipItems.
+    // This will have the important effect of actually returning true in Begin() and not setting SkipItems, allowing an earlier submission of the window contents.
+    // This is analogous to regular windows being hidden from one frame. 
+    // It is especially important as e.g. nested TabBars would otherwise generate flicker in the form of one empty frame, or focus requests won't be processed.
     if (window->DockIsActive && !window->DockTabIsVisible)
     {
-        if (window->LastFrameJustFocused == g.FrameCount) // This may be a better a generalization for the code in BeginDocked() setting the same field.
+        if (window->LastFrameJustFocused == g.FrameCount)
             window->HiddenFramesCannotSkipItems = 1;
         else
             window->HiddenFramesCanSkipItems = 1;
@@ -13527,14 +13531,6 @@ void ImGui::BeginDocked(ImGuiWindow* window, bool* p_open)
     if (node->VisibleWindow == window)
         window->DockTabIsVisible = true;
 
-    // When we are about to select this tab (which will only be visible on the _next frame_), flag it with a non-zero HiddenFramesCannotSkipItems.
-    // This will have the important effect of actually returning true in Begin() and not setting SkipItems, allowing an earlier submission of the window contents.
-    // This is analogous to regular windows being hidden from one frame. It is especially important as nested TabBars would otherwise generate flicker in the form
-    // of one empty frame.
-    // Note that we set HiddenFramesCannotSkipItems=2 because BeginDocked() is called just before Begin() has a chance to decrement the value. Effectively it'll be a 1 frame thing. 
-    if (!window->DockTabIsVisible && node->TabBar && node->TabBar->NextSelectedTabId == window->ID)
-        window->HiddenFramesCannotSkipItems = 2;
-
     // Update window flag
     IM_ASSERT((window->Flags & ImGuiWindowFlags_ChildWindow) == 0);
     window->Flags |= ImGuiWindowFlags_ChildWindow | ImGuiWindowFlags_AlwaysUseWindowPadding | ImGuiWindowFlags_NoResize;