Tooltips and ComboBox dropdown animation stutter on Windows

When clicking on a combobox to display available values, the drop down list stutter when opening itself.

They open with a pause of like 400ms when animating here (the exact moment when it pauses is not exactly the same, the drop down will be a bit more open or closed):

Then after that 500ms pause, the dropdown list continues and open fully.

Also, tooltips don't display itself fully unless I move the mouse:

If I make a small mouse movement, the tooltip displays itself correctly:

It seems that the tooltip animation is blocked and does not resume.

After many tests, I got the tooltip animation blocked like this, which shows the tooltip partially:

See also videos in attachment.

In both cases, it seems that the combobox and tooltip animations are stopping themselves without completing until an event appear (like moving the mouse) which cause them to resume.

If when opening a combobox or a tooltip I continuously move the mouse over the control, the animation is played almost without stutter.

I've tested multiples setups:

• QtCreator 4.12.0 beta2 with Qt 5.14.2: produce the issue state here
• QtCreator 4.10.0 with Qt 5.13.1: no animation issue
• QtCreator 4.10.0 with Qt 5.14.2 (by copying Qt's dll from QtCreator 4.12.0 beta2): produce the issue state here

So it is related to Qt only, not QtCreator.

This is the link I used to install QtCreator 4.12.0 beta2: https://download.qt.io/development_releases/qtcreator/4.12/4.12.0-beta2/qt-creator-opensource-windows-x86_64-4.12.0-beta2.exe