[konsole] [Bug 373232] Horizontal lines with fractional HiDPI scaling

[Ruth Ivimey-Cook]

https://bugs.kde.org/show_bug.cgi?id=373232

Ruth Ivimey-Cook <ruth at ivimey.org> changed:

           What    |Removed                     |Added
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                 CC|                            |ruth at ivimey.org

--- Comment #201 from Ruth Ivimey-Cook <ruth at ivimey.org> ---
I have been seeing the problem as well, using a 4K screen, Ubuntu Focal / x11
on Intel gpu, with scale=1.7 set in kde display settings. AFAIK it doesn't
matter much which scale is used if it's fractional.

I noticed the mention of qt5.15 earlier and so have built qt(head 5.15) and
kde(head master) from latest sources to see if this fixes the problem. I
eventually built the whole of qt except Krita and SignonD. This build is better
compared to kubuntu focal, slightly, but does not fix it (kubuntu shows many
more places with damage, not just konsole).

I did a test just now: in dark-background vim with the cursor at the bottom of
the screen, autorepeat-uparrow to the screen top results in multiple horizontal
lines under the cursor. autorepeat down-arrow tends to clear them, though not
always.

I also often see a vertical line to the right of the cursor when typing at a
prompt, though for some reason this tends not to remain afterwards. It sort of
looks like I've got both a horizontal underline cursor and a vertical one,
though the vertical is one char offset.

With a white window (new firefox tab) behind Konsole, the lines in the konsole
window are white. iconising the window behind means the lines in konsole change
colour to the desktop wallpaper -- i.e. the lines are actually transparent, not
opaque.

I had a look at the relevant code, particularly TerminalDisplay.cpp. There are
a few uses of QRectF in there, which IIRC is a floating point rectangle. I'm
not sure how hidpi works -- does it result in fractional pixel addresses, which
might require floats? Is the problem possibly that more QRects and QPoints in
the code should be QRectF?

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