Long (?) WKT makes delimited text layer fail on 3.16 and 3.18 (works fine on 3.8)

[gerardolm]

Describe the bug
A previously working WKT geometry definition makes "Add delimited text layer" fail. If the record containing the geometry is found at the beginning of the file, no records are added. However, if the record is half-way through the CSV, the records before that one are properly added.

How to Reproduce

1. Get CSV file from https://gist.githubusercontent.com/gerardolm/6c3c8d1fbc53f5f301f2991c3e0c65f3/raw/9d31319023d672b81c224d2cd643c1ecba60d7bd/balears.csv
2. Click on 'Layer', 'Add Layer', 'Add Delimited Text Layer...'
3. Choose the CSV file
4. See error reported at the bottom of the modal screen about "badly formatted records discarded"

QGIS and OS versions

Fails on:

QGIS version	3.18.2-Zürich	QGIS code revision	44d7dfa
Compiled against Qt	5.14.2	Running against Qt	5.14.2
Compiled against GDAL/OGR	3.2.1	Running against GDAL/OGR	3.2.1
Compiled against GEOS	3.9.1-CAPI-1.14.2	Running against GEOS	3.9.1-CAPI-1.14.2
Compiled against SQLite	3.31.1	Running against SQLite	3.31.1
Compiled against PDAL	2.2.0	Running against PDAL	2.2.0 (git-version: Release)
PostgreSQL Client Version	12.3	SpatiaLite Version	4.3.0a
QWT Version	6.1.4	QScintilla2 Version	2.11.4
Compiled against PROJ	6.3.2	Running against PROJ	Rel. 6.3.2, May 1st, 2020
OS Version	macOS 10.15
Active python plugins	QuickWKT; processing; db_manager; MetaSearch

Works fine on previous version:

QGIS version	3.8.1-Zanzibar	QGIS code branch	Release 3.8
Compiled against Qt	5.11.2	Running against Qt	5.11.2
Compiled against GDAL/OGR	2.4.1	Running against GDAL/OGR	2.4.1
Compiled against GEOS	3.7.1-CAPI-1.11.0	Running against GEOS	3.7.1-CAPI-1.11.1 27a5e771
PostgreSQL Client Version	9.6.10	SpatiaLite Version	4.4.0-RC1
QWT Version	6.1.3	QScintilla2 Version	2.10.8
Compiled against PROJ	5.2.0	Running against PROJ	Rel. 5.2.0, September 15th, 2018
OS Version	macOS 10.15

Additional context

Not sure if the problem is the size of the WKT string or some other property of the geometry or the WKT sub-strings.

[gioman]

@gerardolm maybe we are hitting some limit: I cannot copy/paste the (very large) definition of this WKT... not eveb outside QGIS, like in he kate editor (KDE). Opening directly the CSV also causes the editor to lock.