Tooltips for syntax highlights in QTextEdit/QPlainTextEdit

While I was experimenting with adding nlprule-based grammar-checking to my spell-checked QPlainTextEdit subclass (not yet polished enough to share, but there’s a cut-down proof of concept for the nlprule integration on StackOverflow if you want it), I ran into a little problem.

Apparently, there’s an unresolved bug (QTBUG-80524 and duplicates) where QTextEdit and QPlainTextEdit don’t take syntax highlighter-set changes into account when figuring out whether to display a tooltip and what to show.

The solution? Override the lookup behaviour. Here’s how you do it:

class TooltipPlainTextEdit(QPlainTextEdit):
    def __init__(self, *args):
        QPlainTextEdit.__init__(self, *args)

        # Opt in to the events we need to override
        self.setMouseTracking(True)

    def event(self, event) -> bool:
        """Reimplement tooltip lookup to get nlprule messages working"""
        if event.type() == QEvent.ToolTip:
            # Get a secondary text cursor, taking margins into account
            pos = event.pos()
            pos.setX(pos.x() - self.viewportMargins().left())
            pos.setY(pos.y() - self.viewportMargins().top())
            cursor = self.cursorForPosition(pos)

            # QTextCursor doesn't have a quicker way to get
            # highlighter-applied formats, so find the one that our
            # cursor is inside
            for fmt in cursor.block().layout().formats():
                if (fmt.start <= cursor.position() and
                        fmt.start + fmt.length >= cursor.position()):

                    # Calculate the bounding box for the text and feed
                    # it as the rectangle for requesting that the
                    # tooltip auto-hide when the cursor exits
                    cursor.setPosition(fmt.start)
                    cursor.setPosition(fmt.start + fmt.length,
                                       QTextCursor.KeepAnchor)
                    QToolTip.showText(event.globalPos(), fmt.format.toolTip(),
                        self, self.cursorRect(cursor))
                    return True

        # Fall back to the default behaviour
        return super(TooltipPlainTextEdit, self).event(event)

…and that’s it. Your QTextCharFormat.setToolTip calls should now Just Work™.

Tooltips for syntax highlights in QTextEdit/QPlainTextEdit by Stephan Sokolow is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.