Sounds, Videos: a drag changes only the picked clip (#403)

- Sounds: with no clip picked a drag in the strip or the wave makes a
  clip. With one picked its handles move its edges, its band moves it
  whole, and any other drag does nothing. A click in the wave picks.
- Videos: the same gestures, with the track in place of the wave. The
  clips not picked stand faint in the track.
- Development: 1996 editor checks.

Development.md

@@ -21,7 +21,7 @@ php tests/php/releases.test.php                               # the release stor
 php tests/php/files.test.php                                  # uploaded files in a folder per account (#344), the same way
 php tests/php/videos.test.php                                 # the video store, its queue and the worker (#402), with the
                                                               #   real ffmpeg: the CI image and game-maker-video have it
-node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1994 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1996 editor checks, screenshots in /tmp/gm-e2e-*.png
 GM_STEPS=20 node tools/e2e.mjs                                # or 1-12, or 6,20 — a subset while working on one step
 ./e2e.sh start [GM_STEPS]                                     # the same run detached, see below
 ./e2e.sh status                                               # live progress, or the last run's result

Sounds.md

@@ -77,16 +77,21 @@ whatever plays it: `Play sound`, an item in a [song](Songs) and the Sample of an
 counts in it — `length`, the rounds of a song item and their waveform, a Sample's loop points, a
 Pitch.
 
-- **Drag in the strip** to make a new clip from where you pressed. It is picked right away.
-- **Click a band** to pick its clip, **click beside the bands** to pick the sound again. A
-  **click in the wave beside the picked clip** picks the sound too. The picked clip's band is lit,
-  and its stretch stands between **two handles** in the wave. What lies outside is darker. With no
-  clip picked the whole file is bright and has no handles.
+- **With no clip picked, drag** in the strip or in the wave to make a new clip from where you
+  pressed. It is picked right away.
+- **Click a band**, or a clip's stretch in the wave, to pick that clip. **Click beside the clips**
+  to pick the sound again. The picked clip's band is lit, and its stretch stands between **two
+  handles** in the wave. What lies outside is darker. With no clip picked the whole file is bright
+  and has no handles.
 - **The other clips** stay faint in the wave, a pale tint with a thin edge over their stretch, so
   you see where they lie while you work on one.
-- **Drag a handle** to move that edge of the picked clip. **Drag anywhere else** in the wave to
-  span it anew from where you pressed. With no clip picked, a drag in the wave makes a new clip, as
-  in the strip. One drag is one undo step, the two edges together. An edge cannot pass the other.
+- **With a clip picked**, a drag only changes that clip (#403):
+  - **Drag a handle** in the wave to move that edge. An edge cannot pass the other.
+  - **Drag its band** in the strip to move the whole clip. Its length stays, and it stays inside
+    the file.
+  - **Any other drag does nothing.** It makes no new clip and does not span the picked one anew.
+    Pick the sound first to make another clip.
+  - One drag is one undo step.
 - **Reverse**, the button with the play triangle pointing left, plays the picked clip
   [backwards](#playing-it-backwards). It is lit while it is on.
 - **Reverse, the fades and their curves** in the header work on the picked clip and are greyed

Videos.md

@@ -41,11 +41,16 @@ frame rate** and whether it **has sound**, so a sprite is measured before the fi
 
 The video on top, under it a ruler in seconds, the **clip strip** and a **track**:
 
-- **A drag in the strip** (or in the track while no clip is picked) spans a new clip, one undo
-  step, and picks it. **A click on a band** picks that clip, a click beside it in the track picks
-  the video again.
-- The picked clip stands in the track between two handles; **dragging a handle** moves that edge.
-  The picture follows the edge being dragged.
+The clip gestures are the same as in the [Sound tab](Sounds), with the track in place of the wave
+(#403):
+
+- **With no clip picked, a drag** in the strip or in the track spans a new clip, one undo step,
+  and picks it.
+- **A click on a band**, or on a clip in the track, picks that clip. A click beside the clips picks
+  the video again. The clips not picked stand faint in the track.
+- **With a clip picked**, it stands in the track between two handles. **Dragging a handle** moves
+  that edge, **dragging its band** in the strip moves the whole clip, its length kept. **Any other
+  drag does nothing.** The picture follows the edge or the start being dragged.
 - **A click in the ruler** shows that moment.
 - **Play** (or **Space**) plays the picked clip, else the whole video, with its sound and its
   Loop. The playhead runs along the ruler.