Develop Reference

First app page not loading correctly at start, only when app is downloaded from the windows store

I've recently published a windows 8.1 universal app on the Windows Store. When I download it from the store on 8.1 devices, the first page of the app doesn't load correctly, some controls miss. On Windows 10 device, it works well.
If I put the app in the background and then recall it, the page loads correctly.
During the development, I've tested the app on several smartphones and I've never meet this problem.
Has someone met this issue?
In this capture, all controls are loaded
In this one, the checkbox is missing
I'm using the MVVM pattern without external package and I use the control Coding4Fun.Toolkit.Controls.Popup for displaying error messages.

Problem solved, it was a custom control who makes this bug. I use it on each of my pages and it works well. But when the app downloaded from the store starts, it perturbs the loading of the first page of the application.

Windows 8 Touch Multiselect for Gridview vs Listview

I have a windows 8 application that has both list views and grid views. I see the following behavior when multi-selecting items on touch screens:
Grid View: Swipe down on touch screen for selecting items
List view: Swipe left or right for selecting items
Is this the standard behavior? I could not find any documentation on this.
This is for a Windows 8 Store App.

You didn't say whether you were writing WPF or Windows Store Apps, but as always, MSDN is the place to look for documentation either way. For WPF, you have:
ListView Class
GridView Class
And for Windows Store Apps, you have:
ListView Class
GridView Class
If you can't find the information that you need there, then I imagine the next best thing would be to create a simple application without setting any extra properties on your controls and then just try swiping them and then that will be the default behaviour.

Xamarin iOS creating drop down list

I would like to make a Facebook like app which has a dropdown list when clicked on the toolbar button, the view then will have a table view and user can choose one of them then change the view of the MainViewController. So far, I've just added the toolbar items but yet not knowing how to implement the bit that draws a new viewcontroller, please help!

That is not a built-in control in iOS (for iPhone). There is the the popover control for iPad (see here), but that is only for the iPad.
I did a quick search and found FPPopover, but I have not used it before. I'd start there, otherwise what you're looking for is an "implementation of popover control for iPhone".

Make WPF Listview adhere to the Windows theme

I am an experienced coder who is trying to learn C# and WPF. Now, I am a bit of a stickler for detail, and I want to make sure my applications blend in with the rest of the user interface. However, I cannot seem to get this done using a ListView. My setup is simple - add a ListView to my window, bind to an ObservableCollection of my choice, and run the application.
The result is that the items pop up just fine (be it without meaningful contents). However, the selection rectangle on a ListView is always the dark blue that throws me back to the pre-XP days as opposed to the neater themed selection you have using the default Windows 7 visual style. To see that kind of selection in action, just select a file in explorer, or for another example look in the Programs and Features in the Control Panel. That looks nice and slick.
How can I get WPF to properly theme my ListView depending on my user settings? I don't want to reinvent the entire selection code and add tons of styling and doodahs like that - I want a plain Windows application that looks okay. The button works fine, textboxes work fine, comboboxes look fine, etc. It's just the ListView that looks totally crap.
Edit: I have no problems using another (very, very preferably not a thirdparty) control if required, but I do want a list of items that looks native and I approach a layout similar to the Tiles option in Explorer for each selected item (icon, desc, extra line of info). Vertical scrolling only is what I need.

If you use a ListView with ListView.View set to a GridView then you get a pretty similar look as Windows 7 explorer. (Atleast in my opinion, or is this the look you're talking about?). It also follows the active theme
Example 1. ListView with GridView to the right. Selection active
Example 2. ListView with GridView to the right. Selection inactive
Example 3. Using High Contrast Theme #2

Windows mobile UI: Is a messagebox appropriate to display information?

I have a list of items in a listview and when the user double-clicks an item, I display a messagebox with the contact's name and address.
My boss doesn;t like it because he thinks this violate some UI design principle for Windows Mobile devices. I can't find anything that prohibits this. He is thinking that it has something to do with the device not being able to go into power-saving mode when a message-box is displayed.
Is he right? Can you suggest an alternative (he doesn;t have an alternative except he doesn;t like what i have)
C#, Windows Mobile 6.0, WinForms

Windows Mobile 6 uses MessageBox all the time. It is probably better than rolling your own solution because you'll automatically benefit from updates in WM6.5 which a custom solution wouldn't.

The device has no problem going into power-saving mode while a message box is displayed. Other options available to you include using a tabbed view and switching to another tab to display details, or creating and displaying a new form to display details.