in Qt 5.11 there was huge fix for RTL languages as now we properly follow standard.

But when you enable  QTextOption::ShowTabsAndSpaces in document

all space characters is rendered as dots and it change flow of text.

correct flow of text (without ShowTabsAndSpace )

wrong (with ShowTabsAndSpace )

In my text I use LTR mark(just after arabic letters) that change flow of text so 1920 x 1080 is properly rendered

and on this line

http://code.qt.io/cgit/qt/qtbase.git/tree/src/gui/text/qtextengine.cpp#n2066

engine reset bidi(for space) that broke text flow. I read Unicode document and it is right for Tab.

But I don't see any reason to reset bidi for space and Nbsp as we just render something else on place of space character

I made example app that enable/disable ShowTabsAndSpace so problem is visible