WIN32K - A race condition when drawing the titlebar, aka "Spotify eats titlebars"

VBox 4.3.40 with VBEMP
0.4.10-RC-23-g2d74a9f

Reproduction steps
-install Spotify (it will launch automatically after install and will be maximized)
-install Opera 12.18 (it will launch automatically after install and will be windowed)
-activate the apps in alternating order via their taskbar pane

Expected result
They draw correctly including their titlebar

Observed result
-When Opera is activated, it will not draw its titlebar (unless you enforce a refresh explicitly by moving the window)

SpotifyInTheBackground_Prevents_OperasTitlebarToBeDrawn_WhenOperaIsRestored.webm

Contrary OurFileBrowserDoesNot.webm
react.log

Up to here I thought this was 100% reproducible (Spotify eating, explorer eating not).

After experimenting with that for more than an hour, I came to the conclusion that this is a race condition. There is a slight chance that Spotify will not eat up Operas taskbar as well.
I guess the reason for Spotify triggering that so nicely is, that it takes longer to draw its stuff than other apps.

Eventually this relates or is same than CORE-15024, but I can not say for sure.