Qt6: Fix placeholder text color

Freya Lupenrequested to merge
freyalupen/krita:freyalupen/bug-515579 into master

In Qt 5.15, PlaceholderText was made its own ColorRole, but there was a fallback to the old way of 50% alpha of the normal text color. In Qt6 the fallback was removed and then re-added in a different way, but it's seemingly broken, as the PlaceholderText is now 63/255 alpha of black.

We can set the PlaceholderText color ourselves to avoid this. Upstream KColorScheme uses its InactiveText role for this purpose: frameworks/kcolorscheme@d86c9c7f . (That commit's message also suggests something about the way QPalette is constructed being responsible for the PlaceholderText being black.)

However, we don't actually use our copy of KColorScheme::createApplicationPalette, we use ThemeManager::slotChangePalette(). (ThemeManager is from digiKam, which solved this by reimplementing the old way instead, but I think it would be better to eventually resync with KColorScheme.)

I'm not sure how to fix it for our QtQuick widgets.

BUG:515579

Test Plan

  • Check the placeholder text color of things like the searchbars in Settings and resource choosers. It should be readable, especially in dark themes.
  • Check the placeholder text color in the Text Properties Docker of things like 'Add Property' and 'Search'. They're still broken.

Formalities Checklist

  • I confirmed this builds.
  • I confirmed Krita ran and the relevant functions work.
  • I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
  • I made sure my commits build individually and have good descriptions as per KDE guidelines.
  • I made sure my code conforms to the standards set in the HACKING file.
  • I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.
  • Does the patch add a user-visible feature? If yes, is there a documentation MR ready for it at Krita Documentation Repository?

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