Why do custom QWidgets, but not the built-in ones, have nonzero margins?

3
votes

This is a screenshot of a QFormLayout holding a series of widgets. Note that the top and bottom input widgets are not horizontally aligned to the middle two rows. Note also that the labels on the left are not vertically aligned to the middle two rows.

enter image description here

The difference is that the top and bottom widgets are plain QLineEdit and QTextEdit widgets, the second row is a QWidget with a QHBoxLayout holding a QSpinBox and QDateTimeEdit, and the third is a QStackedLayout containing QWidgets each with a QHBoxLayout and API-supplied widgets inside those.

It looks like these middle two have extra margins. I can (partially) improve the situation by calling QLayout::setContentsMargins on the plain QWidgets' layouts. In fact, in the image above I already have, without that it's worse.

I haven't interfered with styles in the application, it's all system-default. This seems to affect Qt5 on Ubuntu 15.04, I don't think I saw this back when it was a Qt4 application.

I tried setStyleSheet ("QWidget {margin:0;}") in the top-level widget, that introduced all sorts of problems. I also tried the variation setStyleSheet (".QWidget {margin:0;}") but that had no effect. QLayout::setSpacing also had no effect, and setting padding:0 in the stylesheet doesn't fix it either.

Nothing I've tried seems to bring the spacing around these custom-layout QWidgets in line with the API-supplied widgets.

What have I missed? Thanks.

1
qtqt5
Have you tried setContentsMargins()? doc.qt.io/qt-5/qlayout.html#setContentsMargins – phyatt
As I said above, yes. It works partially. I'm also wondering why it's needed at all. – spraff

1 Answers

0
votes

What you observe is both the struggle of multi-platform ui and the simple fact that some widgets are horrible. Given a limited amount of time and a big task, dev teams will not treat fairly the set of features. Powerful widgets and layouts like standard widgets, QGridLayout, will be maintained day and night to be defect free while legacy widgets like QStackedWidget will be pushed at the bottom of the maintenance queue.

I tried to reproduce your issue using designer alone and here is what I got (Qt 4.8) :

reproduction of op image

What you can see:

  • The second line with a widget container has great marging
  • The third line doesn't even show the combo box (I swear it's there!!).

My two cents :

  1. There is a difference in behavior in the second row. Because I used Qt4 and you Qt5, and we both set the QHBoxLayout margin to zero then it means there is a potential regression at play, either in Qt5 or the linux implementation. So this confirms what you suspected.
  2. QStackedWidget is horrible. It is the usual widget provided to give quick and basic functionality rather implementing the real deal. Here it would be a widget with a QStackedLayout. Like I said I suspect those kind of widgets to receive less love when big development efforts are required.
  3. In these situations, use QGridLayout instead of a QStackedLayout of widgets with layout of widget. It will solve the issue of labels not being aligned. Difficult to set up at first but the result is usually neat.

By the way, I switched to a widget with a stackedlayout and here is the result : same window with stacked layout