Playblast in newly opened modelPanel

[highend3djojan-]

How can we run playblast in new popup modelPanel by code? I tried this:

import pymel.core as pm

wdw = pm.window()
lyt = pm.frameLayout()
mdl_pnl = pm.modelPanel()
pm.showWindow()

pm.setFocus(mdl_pnl)
pm.rendering.lookThru(mdl_pnl, 'camera2')

pm.playblast(format='avi', filename='C:/Users/J/Desktop/test.avi', showOrnaments=True, forceOverwrite=True,  viewer=True)

... and it creates a playblast but it's of very less height (see the attached) and so I'm not sure if it worked!
Can anybody suggest a workable solution?

Thanks
Jojan

[highend3djojan-]

Note:  Preferences -> Display -> Viewport 2.0 -> Default viewport: Legacy Default viewport  should be set for this code to run without any error.  But still, the playblast is not watchable/good. That's a problem.