Viewports: added io.ConfigViewportPlatformFocusSetsImGuiFocus. (#6299, #6462)

Amend dcb6335bf, 63370be0e, dcb6335 etc.

docs/CHANGELOG.txt

@@ -111,6 +111,12 @@ Docking+Viewports Branch:
   e.g. a ClearActiveID() call would leave the dragged viewport with the
   normally temporary ImGuiViewportFlags_NoInputs flag, preventing further
   interactions with the viewport. (#5324) (thanks @mdelaharpe)
+- Viewports: added io.ConfigViewportPlatformFocusSetsImGuiFocus to opt-out
+  of focusing imgui windows When a platform window is focused (e.g. using Alt+Tab,
+  clicking Platform Title Bar). In principle this is better enabled but we
+  provide an opt-out because some Linux window managers tend to eagerly focus
+  windows (on e.g. mouse hover, or even on a simple window pos/size change).
+  (#6299, #6462)
 
 
 -----------------------------------------------------------------------

imgui.cpp

@@ -1572,6 +1572,7 @@ ImGuiIO::ImGuiIO()
     ConfigViewportsNoTaskBarIcon = false;
     ConfigViewportsNoDecoration = true;
     ConfigViewportsNoDefaultParent = false;
+    ConfigViewportPlatformFocusSetsImGuiFocus = true;
 
     // Miscellaneous options
     MouseDrawCursor = false;
@@ -16384,7 +16385,7 @@ static void ImGui::UpdateViewportsNewFrame()
             // - if focus didn't happen because we destroyed another window (#6462)
             // FIXME: perhaps 'FocusTopMostWindowUnderOne()' can handle the 'focused_window->Window != NULL' case as well.
             const bool apply_imgui_focus_on_focused_viewport = !IsAnyMouseDown() && !prev_focused_has_been_destroyed;
-            if (apply_imgui_focus_on_focused_viewport)
+            if (apply_imgui_focus_on_focused_viewport && g.IO.ConfigViewportPlatformFocusSetsImGuiFocus)
             {
                 focused_viewport->LastFocusedHadNavWindow |= (g.NavWindow != NULL) && (g.NavWindow->Viewport == focused_viewport); // Update so a window changing viewport won't lose focus.
                 ImGuiFocusRequestFlags focus_request_flags = ImGuiFocusRequestFlags_UnlessBelowModal | ImGuiFocusRequestFlags_RestoreFocusedChild;

imgui.h

@@ -2469,6 +2469,7 @@ struct ImGuiIO
     bool        ConfigViewportsNoTaskBarIcon;   // = false          // Disable default OS task bar icon flag for secondary viewports. When a viewport doesn't want a task bar icon, ImGuiViewportFlags_NoTaskBarIcon will be set on it.
     bool        ConfigViewportsNoDecoration;    // = true           // Disable default OS window decoration flag for secondary viewports. When a viewport doesn't want window decorations, ImGuiViewportFlags_NoDecoration will be set on it. Enabling decoration can create subsequent issues at OS levels (e.g. minimum window size).
     bool        ConfigViewportsNoDefaultParent; // = false          // Disable default OS parenting to main viewport for secondary viewports. By default, viewports are marked with ParentViewportId = <main_viewport>, expecting the platform backend to setup a parent/child relationship between the OS windows (some backend may ignore this). Set to true if you want the default to be 0, then all viewports will be top-level OS windows.
+    bool        ConfigViewportPlatformFocusSetsImGuiFocus; //= true // When a platform window is focused (e.g. using Alt+Tab, clicking Platform Title Bar), apply corresponding focus on imgui windows (may clear focus/active id from imgui windows location in other platform windows). In principle this is better enabled but we provide an opt-out, because some Linux window managers tend to eagerly focus windows (e.g. on mouse hover, or even a simple window pos/size change).
 
     // DPI/Scaling options
     // This may keep evolving during 1.92.x releases. Expect some turbulence.

imgui_demo.cpp

@@ -556,6 +556,8 @@ void ImGui::ShowDemoWindow(bool* p_open)
                 ImGui::SameLine(); HelpMarker("Toggling this at runtime is normally unsupported (most platform backends won't refresh the decoration right away).");
                 ImGui::Checkbox("io.ConfigViewportsNoDefaultParent", &io.ConfigViewportsNoDefaultParent);
                 ImGui::SameLine(); HelpMarker("Toggling this at runtime is normally unsupported (most platform backends won't refresh the parenting right away).");
+                ImGui::Checkbox("io.ConfigViewportPlatformFocusSetsImGuiFocus", &io.ConfigViewportPlatformFocusSetsImGuiFocus);
+                ImGui::SameLine(); HelpMarker("When a platform window is focused (e.g. using Alt+Tab, clicking Platform Title Bar), apply corresponding focus on imgui windows (may clear focus/active id from imgui windows location in other platform windows). In principle this is better enabled but we provide an opt-out, because some Linux window managers tend to eagerly focus windows (e.g. on mouse hover, or even a simple window pos/size change).");
                 ImGui::Unindent();
             }