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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.
You open your app in Power Apps Studio and nothing can be changed. The title says (Read-only), the formula bar says Cannot edit in read-only editor, and a banner explains: This app is read-only because you already have editing control elsewhere. Nobody else is editing; "elsewhere" is another session of yours. Override gets you editing again.
The App checker in Power Apps Studio lists red errors: Missing accessible label, one per control. Your screen looks fine, but people using a screen reader get nothing useful from those controls. The fix is to give each one an AccessibleLabel that says what it is or what it does.
The screen looks perfect on your monitor. Then you preview it on a phone, and the text is cut off: half of a heading disappears under the header, and a sentence ends mid-way. Auto height is on, so the text controls are growing. The problem is the container around them, which has a fixed height and doesn't.
