Why did Microsoft think this was a good idea? It seems to keep getting worse with every version of Windows. Let's take a moment to admire the number of different UI concepts you can see in these shots.
There is a traditional file menu on the left although for some reason it's now colored blue like it was selected and what the ribbon is displaying when the ribbon is actually showing home. As an exercise in inconsistency, the file menu is not part of the ribbon, and there are not traditional menus for the ribbon items, so you are forced to use two very different UI paradigms for menus for almost every task. Edit controls are also not part of the ribbon so if you need to undo you'll be using three different UI paradigms.
Hitting alt to select the traditional menu no longer works and now displays a flickering set of floating hotkey letters that vanish and reappear as the mouse moves and stops. This replaces just underlining the 'F' in file, although when expanding ribbon items like paste those items still use underlines. Hitting the menu key if you have one no longer does anything at all.. unless if you're hovering over an item in the file menu where it opens (but not closes) a dialog for adding the item to the quick menu. It does not do this when hovering over ribbon menu items despite it being the same right-click dialog.
The ribbon itself has icons that are grouped and sized by frequency used by Microsoft's testers instead of any logical order ensuring you have to examine them every time you're presented with them as the order differs per application rather than the universal consistency of previous Windows versions. In this case, we have a huge paste icon followed by a small cut then copy.
"Brushes" is a multiple selection and plural, "Size" is a multiple selection and singular. The "Size" icon shows every line that can be selected rather than some indicator of which is selected, but the "Brushes" icon shows only the selected brush.
If you shrink the window vertically all the controls disappear, even the ones that will always have room in the window border. If you shrink it horizontally the ribbon sections collapse into single icons and each is highlighted blue like it is selected, even clipboard which contains no selectable items.. except the size selector which for some reason is not highlighted.
The mouseover tooltips have a bolded one-liner followed by regular text detail, except for color tooltips which only have a one-liner but don't bold like the other tooltips.
And to top it off there's even a set of program functions hoisted into the window border and oddly placed next to the window controls like save, undo, and redo. And just to keep it as inconsistent as possible, undo/redo is paired but open is missing for an open/save pair. Did they not have space for one more icon? Just to make sure no one can ever feel familiar with the UI when using someone else's computer, there's also a customization pulldown that lets you change a few items displayed from the file menu, but not all of them ('save' but no 'save as'). It can even convert this mess into a toolbar in addition to the ribbon.
How does UI this convoluted, confused, and shit get made by a company that supposedly spends billions on UX?