Selectable text in labels

[emilk]

We should have an option to turn on selectable text in labels, i.e. how text works on a website.

Drag-to-select, double-click to select words, triple-click to select entire text, etc.

As a first step, let's limit ourselves to selection within a single Label (not selections spanning multiple widgets).

Since only one label can have a selection at a time, this means some sort of shared mono-state for the labels, mapping a label widget Id to a CCursorRange, similar to TextEdit. We should keep the same selected range of characters even if the label text changes (you should be able to select a label which text is changing).

We need to figure out a good way to share code between TextEdit and Label (all the logic for cursors, selection, etc);

In browsers you can also use the keyboard to select parts of the text (i.e. shift-arrows to move selection), but I suggest we punt on that (I suspect it is a seldom used feature).

Note that making text selectable means they now responds to clicks and drags, so that e.g. "drag-to-scroll" won't work if you drag a label. I think therefore this should be n opt-in feature, globally.

We also have to figure out how this feature interact with text elision (when text is shortened using …)

[YgorSouza]

In web pages, some widgets have selectable text but still respond to click and drag without selecting the text. There are some example right here in this page.

web_browser_text_highlighting.mp4

Would this be an end goal for egui? I guess it would be a matter of keeping a global selection rect for each viewport (or maybe for each area) which is updated according to the user's gestures (Shift + click etc), and then each widget containing selectable text would calculate its intersection with this rect and highlight its text accordingly. And I guess PlatformOutput::copied_text would have to be converted to a Vec? So every widget could push its highlighted text to it, and at the end it would all be joined by newlines and pushed to the clipboard. Or maybe the widgets could just add the newlines themselves as needed when appending to the string.

But egui is also used to make entire desktop apps rather than just web pages, so it should probably still have text that isn't selectable at all, right? Like the tab names and other buttons on the browser.

[emilk]

@YgorSouza that would be nice, but is a lot more workand outside the scope of this issue.

[emilk]

@YgorSouza I created a follow-up issue for multi-widget select in #3816