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QTextLayout: Reconsider cursor drawing on TextObject / Ensure consistent cursor width under fractional scaling

Andreas Sturmlechner requested to merge asturmlechner/qtbase:QTBUG-95319-inconsistent-cursor-w-fractional-scaling into kde/5.15

This one is a bit convoluted, so bear with me. I decided to keep full cherry-pick history including those not slated for 5.15 but it could also be squashed.

Short summary of the commits picked:

  1. e99a883b (has Pick-To 5.15, causing regression)
  2. 33238ea2 (amends 1., not fixing regression)
  3. 3709bc36 (has Pick-To 5.15)
  4. de163006 (rolls back 1. and 2. - touching 3. - but also adds fix for when QScriptItem is a QScriptAnalysis::Object)

The latter was conflicting with 5335cc4a in dev branch, jfyi.

The end result leaves two patch hunks:

a) QTextLayout: Reconsider cursor drawing on TextObject

- Revert: e99a883b
- Revert: 33238ea2

These two commits made the drawing of the cursor incomprehensible,
but their purpose was to fix the problem of abnormal cursor drawing
when QScriptAnalysis::Object is present. Because objects require
some special handling, they can be specially aligned or floated.

[...]

We just need to specially consider the case where the QScriptItem is a
QScriptAnalysis::Object, keeping the base and descent the same as the row.

Task-number: QTBUG-92468
Task-number: QTBUG-86823
Task-number: QTBUG-96288
Pick-to: 6.2 6.4
Change-Id: I6d9a0e00fbc3823e0cc8e0e8bd061da5782d1f8a
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

b) Ensure consistent cursor width under fractional scaling

Under fractional scaling, an N units wide rectangle can in general
cover either M or M+1 pixels, depending on placement. For a tall thin
recangle like the cursor, this difference becomes very visible as the
cursor moves from position to position. Avoid by instead painting the
cursor as a cosmetic line in such cases, since that keeps its width
independently of the current transformation.

Fixes: QTBUG-95319
Pick-to: 6.4 6.3 6.2 5.15
Change-Id: I31a31f89fe7eac3037694946aa452a9f2bd6e5be
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>
Edited by Andreas Sturmlechner

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