A custom Qt FlowLayout that efficiently reorganizes elements when the viewport changes.

FlowLayout.py

import math

from PySide6 import QtWidgets, QtCore


class FlowLayout(QtWidgets.QLayout):

    item_list: list[QtWidgets.QLayoutItem]
    horizontal_space: int
    vertical_space: int
    margin: int

    current_size: tuple[int, int]
    item_width: int
    item_height: int

    def __init__(self, parent: QtWidgets.QWidget, item_width: int, item_height: int):
        super().__init__(parent)

        if parent is not None:
            self.setContentsMargins(0, 0, 0, 0)

        # spaces between each item
        self.horizontal_space = 5
        self.vertical_space = 5
        self.item_width = item_width
        self.item_height = item_height

        self.item_list = []

        self.current_size = (0, 0)

    def __del__(self):
        item = self.takeAt(0)
        while item:
            item = self.takeAt(0)

    def addItem(self, item):
        self.item_list.append(item)

    def count(self) -> int:
        return len(self.item_list)

    def itemAt(self, index) -> QtWidgets.QLayoutItem | None:
        if index >= 0 and index < len(self.item_list):
            return self.item_list[index]

        return None

    def takeAt(self, index: int) -> QtWidgets.QLayoutItem | None:
        if index >= 0 and index < len(self.item_list):
            return self.item_list.pop(index)

        return None

    def addWidget(self, widget: QtWidgets.QWidget):
        super().addWidget(widget)

    def expandingDirections(self) -> QtCore.Qt.Orientations:
        return QtCore.Qt.Orientations(QtCore.Qt.Orientation(0))

    def hasHeightForWidth(self) -> bool:
        return True

    def heightForWidth(self, width: int) -> int:
        if width == 0:
            return -1

        # TODO take into account the right most horizontal_space
        column_count = max([width // self.item_width, 1])
        row_count = math.ceil(len(self.item_list) / column_count)

        height = row_count * (self.item_height + self.horizontal_space)

        # if even, remove last spacing
        if len(self.item_list) % 2 == 0:
            height -= self.vertical_space

        return height

    def setGeometry(self, rect: QtCore.QRect):

        super().setGeometry(rect)
        self.doLayout(rect)

    def sizeHint(self) -> int:
        return self.minimumSize()

    def minimumSize(self) -> int:
        size = QtCore.QSize()

        for item in self.item_list:
            size = size.expandedTo(item.minimumSize())

        return size

    def doLayout(self, rect: QtCore.QRect):

        if (rect.width(), rect.height()) == self.current_size:
            return

        self.current_size = (rect.width(), rect.height())
        column_count = max([rect.width() // self.item_width, 1])

        centering_offset = (rect.width() - (column_count * self.item_width)) // 2

        row = 0
        column = 0
        for item in self.item_list:
            x_offset = column * self.item_width + centering_offset
            y_offset = row * self.item_height

            column += 1
            if column == column_count:
                column = 0
                row += 1

            item.setGeometry(QtCore.QRect(x_offset, y_offset, self.item_width, self.item_height))