Agat Emulator 1.29.1, titlebar is not redrawn when switching back from fullscreen

VBox 4.3.40

0.4.10-RC-22-gd3cc081 gcc dbg linux + Agat Emulator 1.29.1 + VBEMP is affected.

I noticed this already with older Agat Emulator 1.27 in 0.4.7, 0.4.8 & 0.4.9 release tests. So nothing new.

Reproduction steps
-Open Agat Emulator
-Run a simulation machine
-switch to fullscreen via alt+enter
-switch back to windowed via alt+enter

Expected result
-the windows titlebar and borders are drawn

Observed result
-the windows titlebar and borders are not drawn, but when you wiggle the window a bit, a redraw is enforced

0.4.10-RC-22-gd3cc081__vs_Agat2_9_1.log
0.4.10-RC-22-gd3cc081__vs_Agat2_9_1.webm