Bug: 

• Strong and permanent flickering when draging frameless window between monitors with different dpis

Fixture

• two Monitors, one left, one right where you can move windows from one monitor to the other 
• left monitor: normal dpi with 100% scaling
• right monitor: high dpi with 150% scaling

Used Application attributes

• AA_EnableHighDpiScaling

Application specs

• Frameless QMainWindow with central widget
• Central widgte has a close button and a drag label
• drag label uses event filter to move the windows around
• event filter reacts on Mouse press/move/release events
• event filter moves main window

Reproduction steps (move via event filter)

• build application (see attached source)
• Click and hold on Label ("Grab me and move me around") on the most right side
• drag window slowly on the left monitor

Result

1. a strong flickering occurs while the window is permamently repositioned till it reachs the left monitor
2. we get permamently errors from QWindowsWindow::setGeometry till the left monitor is reached

Variations

use QWidget instead of QMainWindow and move via event filter

• comment line 79 in Application.cpp
• uncomment line 80 in Application.cpp
• build
• Click and hold on Label ("Grab me and move me around") on the most right side
• drag window slowly on the left monitor

Result

• a strong flickering occurs while the window is permamently repositioned till it reachs the right monitor
• we get permamently errors from QWindowsWindow::setGeometry till the left monitor is reached

Disabled FramelessWindowHint and move via event filter

• comment line 77 in Application.cpp
• uncomment line 79 in Application.cpp
• comment line 80 in Application.cpp
• build
• Click and hold on Label ("Grab me and move me around") on the most right side
• drag window slowly on the left monitor

Result

• no flickering, but wrong position while transition from right to left monitor
• we get permamently error from QWindowsWindow::setGeometry till the left monitor is reached

Disable AA_EnableHighDpiScaling, Enabled or Disabled FramelessWindowHint and via event filter

• comment line 7 main.cpp
• un/comment line 77 in Application.cpp
• uncomment line 79 in Application.cpp
• comment line 80 in Application.cpp
• build
• Click and hold on Label ("Grab me and move me around") on the most right side
• drag window slowly on the left monitor

Result

• no flickering
• one error from QWindowsWindow::setGeometry

Move with Titlebar

• comment line 77 in Application.cpp
• uncomment line 79 in Application.cpp
• comment line 80 in Application.cpp
• build
• Click and hold on Titlebar on the most right side
• drag window slowly on the left monitor

Result

• correct positioning
• no errors