Question

So why does setting alternatingRowColors cause the change in highlight?

Qt tries to use the OS default widgets first. Alternating row colors is not supported by the OS directly. So, Qt will draw it from scratch and usually tries to mimic the OS look. Seems like it did not get updated, yet.

So why does setting alternatingRowColors cause the change in highlight? Is that a bug?

The WinUI3 style has no information about alternating row colors so this is the solution for this.

med up against the left side of the text with no intervening space

This will be fixed

I'm not trying to be difficult. I'm still learning Qt/PySide6 (excellent tools IMO) and am puzzled by the need to set alternatingRowColors just to get the expected highlight behavior.

Notice the "selection" highlight in the second screenshot is just a thin blue line at the left of each cell.

And that's the correct winui3 style.

@Jim-G said in Selection highlight is a thin blue bar at the left of the cell.:

Bug?

No, it's the default WinUI 3 style. Look into your explorer to see that the highlight is gray.

I'm not following. I think we're talking about two different things. I can reproduce the issue using the sample code only.

Here's the UI including self.table_view.setAlternatingRowColors(True) and line 4 selected:

And now with that single line commented out and line 3 selected:

Notice the "selection" highlight in the second screenshot is just a thin blue line at the left of each cell.

It is unexpected that displaying the correct highlighting would involve the setting of alternatingRowColors.

Bug?

No, it's the default WinUI 3 style.

Other than the change of model class, the only other difference I could find was that in the sample code there is this:

        # Alternating row colors for data density scannability
        self.table_view.setAlternatingRowColors(True)

So I tried adding that to my code, and it fixed the issue:

Bug?

Also note that in my earlier screenshot showing the problem, there is already alternate row highlighting which is unexpected as the default for alternatingRowColors is False. Something's amiss here.

Using return super().data(index,role) raises NotImplementedError as expected

You data method only returns something for the few roles you have specified. Otherwise you are returning None. Try instead to return QAbstractItemModel.data(...) (I don't know Python and can't tell you how to exactly write that). This might solve your problem, but I'm not sure.

    return super().data(index, role)

I tried some sample code using QTableWidget, and that works correctly. I then made sure my code sets all the same attributes on the view and the model, with no effect. I'm stumped. I guess I'm going to have to reimplement my code with QTableWidget and see what happens.

More later...

As ever with this sort of thing, try to reduce to minimal and then build up.

I don't recall ever having a problem with selection highlighting nor doing anything at all about font, font metrics or sizes. If I were you I would:

• Temporarily try a QTableWidget. Put in a couple of fixed rows and a couple of columns. Try a couple of SelectionBehaviours. Works or looks odd?

• Move to a QTreeView but try a QStandardItemModel for the model. Again put in a couple of rows and columns. How does selection look?

If problems there that is one thing. If they work but yours does not then start looking at your implementation.

P.S.
Is your issue because you did not implement the flags() override? You may need to do that for a QAbstractTableModel. You need to return at least ItemIsSelectable: because you have not I wonder of your items are just not selectable so it won't show properly? Check what your current SAEventsModel.flags(index) is returning. It actually ought be correct without override, but just verify.

QAbstractTableModel Subclass

class ColDef:
    ''' 
        Store column definitions for a model
        
        Still a work-in-progress 
    '''
    def __init__(self, header:str, attr:str, fmt:str, align:Qt.AlignmentFlag):
        self.header = header
        self.attr   = attr
        self.fmt    = fmt
        self.align  = align
        
    def format(self, row) -> str:
        if self.attr == '': return ''
        val = getattr(row, self.attr)
        
        if self.fmt == 'time':
            if isinstance(val, dt.datetime):
                return val.strftime("%H:%M")
        else: # For now...
            return str(val)
        
        return None

columnDefs = [
    #       Header              Field in rowset                 Format  Alignment
    #       ------------------  ------------------------------  ------- ---------------
    ColDef('Scheduled\nStart', 'start_ts',                      'time', Qt.AlignCenter),
    ColDef('Scheduled\nEnd',   'end_ts',                        'time', Qt.AlignCenter),
    ColDef('Leader',           '',                              '',     Qt.AlignLeft),
    ColDef('Actual\nStart',    '',                              '',     Qt.AlignCenter),
    ColDef('Actual\nEnd',      '',                              '',     Qt.AlignCenter),
    ColDef('Activity',         '',                              '',     Qt.AlignCenter),
    ColDef('Description',      'title',                         '',     Qt.AlignLeft),
    ColDef('Limit',            'registrations_limit',           'int',  Qt.AlignCenter),
    ColDef('Reg',              'confirmed_registrations_count', 'int',  Qt.AlignCenter),
    ColDef('Wait',             'waitlist_registration_count',   'int',  Qt.AlignCenter),
]

class SAEventsModel(QAbstractTableModel):
    def __init__(self, parent=None, * , dbEngine):
        super().__init__()
        
        self.dbEngine = dbEngine
        self.rowset   = []
        self.fontMetrics = None

    def setFontMetrics(self, font:QFont):
        self.fontMetrics = QFontMetrics(font)

    def refresh(self, d:dt.date):
        self.beginResetModel()
        
        with Session(self.dbEngine) as dbSession:
            stmt = select(DVEventSession)\
                    .where(DVEventSession.event_date==d)\
                    .order_by(DVEventSession.start_ts)
            self.rowset = dbSession.scalars(stmt).all()
            
        self.endResetModel()

    def headerData(self, section:int, orientation:Qt.Orientation, role:Qt.ItemDataRole):
        if orientation == Qt.Orientation.Horizontal \
                and 0 <= section < len(columnDefs) \
                and role == Qt.DisplayRole:
            return columnDefs[section].header
        return None

    def rowCount(self, _) -> int:
        return len(self.rowset)

    def columnCount(self, _) -> int:
        return len(columnDefs)
    
    def data(self, index:QModelIndex, role:Qt.ItemDataRole):
        if index.isValid():
            row = index.row()
            col = index.column()
            cd = columnDefs[col]
            
            if role in (Qt.ItemDataRole.DisplayRole, Qt.ItemDataRole.EditRole):
                # Table is read-only, return same data for Display and Edit
                return cd.format(self.rowset[row])
            
            elif role == Qt.ItemDataRole.SizeHintRole:
                # Recalculate the text size 
                text = cd.format(self.rowset[row])
                w    = self.fontMetrics.horizontalAdvance(text)
                h    = self.fontMetrics.height()
                return QSize(w+20, h+10)
            
            elif role == Qt.ItemDataRole.TextAlignmentRole:
                # Return the desired alignment
                return cd.align
            
        return None

Main Window snippet

self.ui is loaded from a QtCreator-generated .ui file. self.ui.guildEvents' is a QTableView`

        td = dt.date.today()
        self.ui.curDate.setText(f"{td:%B} {td.day} {td:%Y}")
        
        self.model = SAEventsModel(dbEngine=dbEngine)
        self.model.refresh(td)
        self.model.setFontMetrics(self.ui.guildEvents.font())
        
        self.ui.guildEvents.setModel(self.model)
        self.ui.guildEvents.horizontalHeader().setSectionResizeMode(QHeaderView.ResizeMode.ResizeToContents)
        self.ui.guildEvents.setSelectionBehavior(qv.SelectionBehavior.SelectRows)

Main Window Display

The middle row is selected.