建议先看另一篇blog：
https://blog.csdn.net/learnframework/article/details/123032419

系统如何让桌面执行对应的onStart方法呢？

具体的堆栈显示如下：

makeActiveIfNeeded:5788, ActivityRecord (com.android.server.wm)
makeVisibleIfNeeded:5697, ActivityRecord (com.android.server.wm)
setActivityVisibilityState:210, EnsureActivitiesVisibleHelper (com.android.server.wm)
process:143, EnsureActivitiesVisibleHelper (com.android.server.wm)
updateActivityVisibilities:1117, TaskFragment (com.android.server.wm)
lambda$ensureActivitiesVisible$18:4857, Task (com.android.server.wm)
accept:-1, Task$$ExternalSyntheticLambda23 (com.android.server.wm)
forAllLeafTasks:3174, Task (com.android.server.wm)
forAllLeafTasks:3162, Task (com.android.server.wm)
ensureActivitiesVisible:4856, Task (com.android.server.wm)
lambda$ensureActivitiesVisible$45:6293, DisplayContent (com.android.server.wm)
accept:-1, DisplayContent$$ExternalSyntheticLambda38 (com.android.server.wm)
forAllRootTasks:3186, Task (com.android.server.wm)
forAllRootTasks:2014, WindowContainer (com.android.server.wm)
forAllRootTasks:2014, WindowContainer (com.android.server.wm)
forAllRootTasks:2014, WindowContainer (com.android.server.wm)
forAllRootTasks:2014, WindowContainer (com.android.server.wm)
forAllRootTasks:2014, WindowContainer (com.android.server.wm)
forAllRootTasks:2014, WindowContainer (com.android.server.wm)
forAllRootTasks:2007, WindowContainer (com.android.server.wm)
ensureActivitiesVisible:6292, DisplayContent (com.android.server.wm)
ensureActivitiesVisible:1864, RootWindowContainer (com.android.server.wm)
ensureActivitiesVisible:1845, RootWindowContainer (com.android.server.wm)
startRecentsActivity:260, RecentsAnimation (com.android.server.wm)
startRecentsActivity:1729, ActivityTaskManagerService (com.android.server.wm)
onTransact:1171, IActivityTaskManager$Stub (android.app)
onTransact:5183, ActivityTaskManagerService (com.android.server.wm)
execTransactInternal:1280, Binder (android.os)
execTransact:1244, Binder (android.os)

导致onStart调用关键就在于这里的startRecentsActivity会触发ensureActivitiesVisible方法，正常的ensureActivitiesVisible只是来保证当前一些ActivityRecord的属性值或者config变化了，靠调用ensureActivitiesVisible来保证Activity的visibility可以显示正常

这里的
targetActivity.mLaunchTaskBehind = true;
就是最为关键的点，它代表者可以在其他activity的后面可以显示。

疑问为啥只是到了onStart而不走到onResume：

根据上面的堆栈来看这个核心方法makeActiveIfNeeded

boolean makeActiveIfNeeded(ActivityRecord activeActivity) {
//判断Activity是应该被Resume
        if (shouldResumeActivity(activeActivity)) {
            return getRootTask().resumeTopActivityUncheckedLocked(activeActivity /* prev */,
                    null /* options */);
        } else if (shouldPauseActivity(activeActivity)) {
//判断Activity是应该被Pasue
        
            setState(PAUSING, "makeActiveIfNeeded");
            mAtmService.getLifecycleManager().scheduleTransaction(app.getThread(), token,
                        PauseActivityItem.obtain(finishing, false /* userLeaving */,
                                configChangeFlags, false /* dontReport */));

        } else if (shouldStartActivity()) {
         
            setState(STARTED, "makeActiveIfNeeded");

            try {
                mAtmService.getLifecycleManager().scheduleTransaction(app.getThread(), token,
                        StartActivityItem.obtain(takeOptions()));
            } 
            mTaskSupervisor.mStoppingActivities.remove(this);
        }
        return false;
    }

那么这里其实最主要我们关注是shouldResumeActivity(activeActivity)方法为啥没有返回true，而返回来false，导致进入到了shouldStartActivity方法
看看shouldResumeActivity方法：

  @VisibleForTesting
    boolean shouldResumeActivity(ActivityRecord activeActivity) {
    //主要调用到了shouldBeResumed
        return shouldBeResumed(activeActivity) && !isState(RESUMED);
    }
    private boolean shouldBeResumed(ActivityRecord activeActivity) {
    //shouldBeResumed方法又有若干条件，任何条件不满足都是不可以的，先看看第一个条件shouldMakeActive
        return shouldMakeActive(activeActivity) && isFocusable()
                && getTaskFragment().getVisibility(activeActivity)
                        == TASK_FRAGMENT_VISIBILITY_VISIBLE
                && canResumeByCompat();
    }
    
  boolean shouldMakeActive(ActivityRecord activeActivity) {
        if (!isState(STARTED, RESUMED, PAUSED, STOPPED, STOPPING)
                // TODO (b/185876784) Check could we remove the check condition
                //  mTranslucentActivityWaiting != null here
                || getRootTask().mTranslucentActivityWaiting != null) {
            return false;
        }

        if (this == activeActivity) {
            return false;
        }

        if (!mTaskSupervisor.readyToResume()) {
            // Making active is currently deferred (e.g. because an activity launch is in progress).
            return false;
        }
//核心就是这个地方，前面设置了mLaunchTaskBehind为true，所以这里就返回false了
        if (this.mLaunchTaskBehind) {
            // This activity is being launched from behind, which means that it's not intended to be
            // presented to user right now, even if it's set to be visible.
            return false;
        }
        return getTaskFragment().topRunningActivity() == this;
    }