Microsoft Power Apps Canvas Apps, Power Fx Formulas, and Controls
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A horizontal container lines up the controls inside it from left to right and spreads them across the space it has. Placed inside a vertical container, it's the natural way to build a header bar, a row of buttons, or a picture next to some text. Here's how to add one and give it a fixed height, using an app header as the example.
A vertical container stacks the controls inside it, top to bottom, and works out their positions for you. You never type an X or Y for them. Put one around a whole screen and set it to the screen's size, and everything inside follows when the window gets wider or narrower. It's the starting point of a responsive canvas app.
Good names make a canvas app readable: btnMyPlanHS tells you it's a button on the Home screen, Button4 tells you nothing. Microsoft publishes naming guidance for Power Apps, and following it costs nothing but a rename. Here it is in one place, with the Career Development Plan app as a worked example.
Every new screen and control gets a placeholder name: Screen1, Container1, Button1. Those names end up in your formulas, and screen names are read aloud by screen readers. Renaming takes a few seconds, and doing it as soon as you add something saves confusion later.
By default, a canvas app opens on the first screen in the tree view. App.StartScreen lets you say which screen opens first, on purpose and in one place. It replaces putting Navigate in App.OnStart, which Microsoft has retired because it slows loading.
A named formula gives a name to a formula so you can use the name anywhere in your canvas app. It replaces the old habit of setting global variables in App.OnStart. The value is always up to date, nothing can overwrite it by accident, and Power Apps only works it out when something needs it.
Power Apps Studio has a setting called Enable App.OnStart property, with a warning that OnStart "can cause delays when loading an app". It's tempting to switch it off to keep an app fast. This guide tested what the switch actually changes, and the result is a little surprising: in Studio, an OnStart formula still ran with the switch off.
Modern controls follow your app's theme on their own. Containers and classic controls don't: their colors and fonts stay whatever you set. The fix is to point those properties at the theme itself with App.Theme, so a header bar or a title label changes along with everything else when you switch themes.
A modern theme sets the colors and font of every modern control in a canvas app at once. Pick a theme, and buttons, avatars and anything else that follows the theme change together. Here's where the themes are, what happens when you pick one, and how to answer the question Studio asks about classic controls.
Modern controls are the newer set of Power Apps controls, built on Microsoft's Fluent 2 design system. They look current, work better with screen readers and keyboards, and all take their colors from one theme. In some apps they're switched off, and the Insert pane only offers the older, classic controls. Here's how to turn them on.
