fix blurry favicons on hidpi displays

[foliea]

This fixes blurry favicons on hidpi displays.

Related to #1585

Results on non hidpi display:

Results on hidpi display:

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This change is 

[The-Compiler]

Thank you! It'll probably take me a bit until I get to properly reviewing this, as I was really busy with v1.0 and the attention it has received over the past few days.

FWIW the docs say:

Make QIcon::pixmap() generate high-dpi pixmaps that can be larger than the requested size. Such pixmaps will have devicePixelRatio() set to a value higher than 1. After setting this attribute, application code that uses pixmap sizes in layout geometry calculations should typically divide by devicePixelRatio() to get device-independent layout geometry.

The only place qutebrowser uses QIcon::pixmap is here in tabwidget.py:

        icon = opt.icon.pixmap(opt.iconSize, icon_mode, icon_state)
        p.drawPixmap(layouts.icon.x(), layouts.icon.y(), icon)

so this should be fine.

@jgkamat since you looked at tab sizing stuff recently, want to take a look?

[jgkamat]

I took a look at the logic, and I don't think this will break anything. I don't have a HiDPI monitor, so I can't test this too deeply though. It does work for me as expected on my computers.

Could you do me a favor and make sure pinned tab's widths aren't cut off or too long with this patch? I don't suspect it will make a difference though.

[dvs1]

Pinning tabs seems the same on this iMac.

Regardless of pinning, depending on the website, this change can invoke a different favicon.
That's not a problem for me, I just thought it was worth mentioning.

[dvs1]

For example, I get the "For Apple devices" icon with this patch,
and "All others" without it.

<!-- favicon -->
<!-- For Apple devices -->
<link href='/assets/apple-touch-icon-9f7c9cc3923b4817fbb25bfeb0dd5fd34638827fd9c82b5a03f7b68e6e90806f.png' rel='apple-touch-icon'>
<!-- For Nokia devices -->
<link href='/assets/apple-touch-icon-9f7c9cc3923b4817fbb25bfeb0dd5fd34638827fd9c82b5a03f7b68e6e90806f.png' rel='shortcut icon'>
<!-- All others -->
<link href='/assets/favicon-6c483382d6196fb46215c44e6bdaffcdd019cc0a8e12fd1a1b62356721de1476.png' rel='shortcut icon'>

source: https://diasp.org

[The-Compiler]

For example, I get the "For Apple devices" icon with this patch, and "All others" without it.

I can't reproduce this (Linux, Qt 5.9, both QtWebKit and QtWebEngine) - I get the normal (red star on black circle) icon both with and without the patch.

[nicktelford]

I get the white star on black background "for Apple devices" icon (Linux, Qt 5.9, QtWebEngine, 4K High DPI display).

I suspect it's actually selecting the "Nokia devices" icon, as that's the same PNG, but with the shortcut icon type. I think it's selecting the higher resolution icon if the screen resolution is high enough to warrant it.

[The-Compiler]

Hmm, seeing that this seems to display favicons which aren't really suitable for qutebrowser's small default favicon size, I feel like this should be an option. @foliea is this something you'd like to work on? Let me know if you need help.

[dvs1]

...aren't really suitable for qutebrowser's small default favicon size...

In what sense? They are scaled to be the same size in qutebrowser.

[The-Compiler]

I don't follow - from what it sounds, qutebrowser show the icon intended for high resolutions as something like a 10x10px icon, right?

[dvs1]

The icon seems to scale with c.fonts.tabs, so that it is always the appropriate size.

With the default:

With 14pt:

[The-Compiler]

yes - what I'm saying is that e.g. https://diasp.org/ has two favicons - a small one:

and a bigger one (for phone launchers?):

If I'm understanding what you said above correctly, you get the bigger one, but scaled down to 10x10 or so? If that's happening with this patch, I think it should be using a setting.

[dvs1]

Actually, in testing just now, it appears that c.fonts.tabs being set to 14pt is what triggers the alternative icon.

Default:

14pt:

[The-Compiler]

Ah, that seems fine to me then - thanks everyone for the PR, reviews and tests!