Expand/collaps icon in QTreeView is not correctly scaled when multi monitors have different scale settings

Issue:
A QTreeView application does not scale the expand/collaps icon when it runs with multiple monitors that have different scale settings.
That is more clearly observed when the difference between scale settings is large.

Steps to reproduce:
1. Prepare a laptop and an external monitor
2. Set the laptop's built-in monitor as a main monitor, and set its scale to 250% (or a maximum value)
3. Set the external monitor as a sub monitor, and set its scale to 100%
4. Run the attached application.

• If the application appears in the main monitor first, its expand icon will be scaled correctly. That will be kept even you drag the window to the sub monitor.
• If the application appears in the submonitor first, its expand icon will be scaled larger than expected. 
  The behavior may varies on scale setting.

The deviation becomes clearer if you enlarge the difference between two scale settings.
For example setting the environment variables as follows makes it more noticeable :
    qputenv("QT_SCREEN_SCALE_FACTORS", "5;1");